path: root/shared-mime-info-spec.xml

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  <!ENTITY updated "05 Mar 2003">
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<article id="index">

<articleinfo>
	<authorgroup>
		<corpauthor>
			<ulink url="http://www.freedesktop.org">
			X Desktop Group
			</ulink>
		</corpauthor>
		<author>
			<firstname>Thomas</firstname>
			<surname>Leonard</surname>
			<affiliation>
				<address><email>tal197@users.sf.net</email></address>
			</affiliation>
		</author>
	</authorgroup>

	<title>Shared MIME-info Database</title>
	<date>&updated;</date>
</articleinfo>

<sect1>
	<title>Introduction</title>
	<sect2>
		<title>Version</title>
		<para>
This is version &version; of the Shared MIME-info Database specification, last updated &updated;.</para>
	</sect2>
	<sect2>
		<title>What is this spec?</title>
		<para>
Many programs and desktops use the MIME system<citation>MIME</citation>
to represent the types of files. Frequently, it is necessary to work out the
correct MIME type for a file. This is generally done by examining the file's
name or contents, and looking up the correct MIME type in a database.
		</para>
		<para>
It is also useful to store information about each type, such as a textual
description of it, or a list of applications that can be used to view or edit
files of that type.
		</para>
		<para>
For interoperability, it is useful for different programs to use the same
database so that different programs agree on the type of a file and
information is not duplicated. It is also helpful for application authors to
only have to install new information in one place.
		</para>
		<para>
This specification attempts to unify the MIME database systems currently in
use by GNOME<citation>GNOME</citation>, KDE<citation>KDE</citation> and
ROX<citation>ROX</citation>, and provide room for future extensibility.
		</para>
		<para>
The MIME database does NOT store user preferences (such as a user's preferred
application for handling files of a particular type). It may be used to store
static information, such as that files of a certain type may be viewed with
a particular application.
		</para>
	</sect2>
	<sect2>
		<title>Language used in this specification</title>
		<para>
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this document are to be
interpreted as described in RFC 2119<citation>RFC-2119</citation>.
		</para>
	</sect2>
</sect1>
<sect1>
	<title>Overview of previous systems</title>
	<sect2>
		<title>KDE</title>
		<para>
KDE uses <filename>.desktop</filename> files, with Type=MimeType, one file
per type to determine type from file name. The files are arranged in the
filesystem to mirror the two-level MIME type hierarchy.
The syntax is very similar to other <filename>.desktop</filename> files,
with Name=, Comment= etc.
		</para>
		<para>
Example file:
			<programlisting><![CDATA[
[Desktop Entry]
Encoding=UTF-8
MimeType=application/x-kword
Comment=KWord
Comment[af]=kword
[... etc. other translations ]
Icon=kword
Type=MimeType
Patterns=*.kwd;*.kwt;
X-KDE-AutoEmbed=false

[Property::X-KDE-NativeExtension]
Type=QString
Value=.kwd
]]></programlisting>
		</para>
		<para>
KDE does not have a separate system for specifying extension matches, but
uses case-sensitive glob patterns for everything.
		</para>
		<para>
A single file stores all the rules for recognising files by content. This
is almost identical to <citerefentry><refentrytitle>file</refentrytitle>
<manvolnum>1</manvolnum></citerefentry>'s <filename>magic.mime</filename>
database file, but without the encoding field.
		</para>
		<para>
The format is described in the file itself as follows:
		<programlisting><![CDATA[
# The format is 4-5 columns:
#    Column #1: byte number to begin checking from, ">" indicates continuation
#    Column #2: type of data to match
#    Column #3: contents of data to match
#    Column #4: MIME type of result
]]></programlisting>
		</para>
	</sect2>
	<sect2>
		<title>GNOME</title>
		<para>
GNOME uses the gnome-vfs library to determine the MIME type of a file.
This library loads name-to-type rules from files with a '.mime' extension
in a system-wide directory (set at install time), and merged with those in the
user's directory. It loads textual descriptions for the types from
files in the same directories, ending with '.keys'. The file
<filename>gnome-vfs.mime</filename> in the system directory is always loaded
first (allowing everything else to override it). The file
<filename>user.mime</filename> in the user's directory is always loaded
last, making these settings take precedence over all others.
		</para>
		<para>
The format of the .mime files are described as follows:
			<programlisting>
# Mime types as provided by the GNOME libraries for GNOME.
#
# Applications can provide more mime types by installing other
# .mime files in the PREFIX/share/mime-info directory.
#
# The format of this file is:
#
# mime-type
#	ext[,prio]:	list of extensions for this mime-type
#	regex[,prio]:	a regular expression that matches the filename
#
# more than one ext: and regex: fields can be present.
#
# prio is the priority for the match, the default is 1. This is required
# to distinguish composed filenames, for example .gz has a priority of 1
# and .tar.gz has a priority of 2 (thus a file having the filename
# something.tar.gz will match the mime-type for tar.gz before the mime-type
# for .gz
#
# The values in this file are kept in alphabetical order for convenience.
# Please maintain this when adding new types. Also consider adding a
# human-readable description to gnome-vfs.keys when adding a new type here.
#
# Also do please not add illegal mime types, observe the mime standard when
# adding new types.
			</programlisting>
When looking up the type for a file, gnome-vfs looks first for an exact-case
match for the extension, then an all upper-case match, then an all lower-case
match. If no matches are found, or there is no '.' in the name, then the
regular expression matches are checked. It does this first for rules with
priority 2, then for those with priority 1. The modification time on the
<filename>mime-info</filename>
directories is used to detect changes.
		</para>
		<para>
The .keys files contain type-to-description rules, eg:
			<programlisting>
application/msword
	description=Microsoft Word document
	[de]description=Microsoft Word-Dokument
	...
			</programlisting>
Guidelines for writing descriptions can be found in the
<filename>mime-descriptions-guidelines.txt</filename> file.
		</para>
		<para>
The format for magic entries is defined as:
			<programlisting><![CDATA[
# The format of magic entries is:
#
#     offset_start[:offset_end] pattern_type pattern [&pattern_mask] type
#
# <offset_start> and <offset_end> are decimal numbers (file offsets).
#
# <pattern_type> is (byte | short | long | string | date | beshort |
#                    belong | bedate | leshort | lelong | ledate).
#
# <pattern> is an ASCII string with non-printable characters escaped
# as hex or octal escape sequences, and spaces and other important
# whitespace escaped with '\'.
#
# <pattern_mask> is a string of hex digits. The mask must be the same
# length as the pattern.
#
# <type> is a valid MIME type.
#
# Order magic patterns such that ambiguous ones (such as
# application/x-ms-dos-executable) are at the end of the list and
# therefore get applied last.
#
# Avoid rules that require a seek deep into the examined file. If you
# must, locate such rules at the end of the list so that they get
# applied last
#
# When designing new document formats, make them easily recognizable
# by defining a sufficiently unique magic pattern near the document
# start. A good pattern is at least four bytes long and contains one
# or two non-printable characters so that text files won't be
# misidentified.
]]></programlisting>
		</para>
	</sect2>
	<sect2>
		<title>ROX</title>
		<para>
Note that ROX is now using this specification. This section details the
previous implementation.
		</para>
		<para>
ROX searches <filename>MIME-info</filename> directories in
<envar>CHOICESPATH</envar> (<filename>~/Choices/MIME-info:/usr/local/share/Choices/MIME-info:/usr/share/Choices/MIME-info</filename> by
default). Files from earlier directories override those in later ones, but
the order within a directory is not specified.
		</para>
		<para>
The files are in the same format as GNOME, except:
			<itemizedlist>
				<listitem><para>
There are no .keys files, so files of all extensions are loaded.
				</para></listitem>
				<listitem><para>
The priority is ignored.
				</para></listitem>
				<listitem><para>
A case-sensitive match is tried first, then a lower-case match. No upper-case
match is tried.
				</para></listitem>
				<listitem><para>
Multiple extensions are allowed. Eg:
					<programlisting>
application/x-compressed-postscript
	ext: ps.gz eps.gz
					</programlisting>
				</para></listitem>
			</itemizedlist>
		</para>
		<para>
When looking up the type for a file, ROX starts with the first '.'
and tries a case-sensitive match of the remaining text against the extensions.
The it tries again with the filename in lower-case. It then tries again
from the second '.', and so on. If no type is found, it tries the regular
expressions.
		</para>
		<para>
ROX has no rules for determining a file's type from its contents.
		</para>
	</sect2>
</sect1>



<sect1>
	<title>Unified system</title>
	<para>
In discussions about these systems, it was clear that the differences between
the databases were simply a result of them being separate, and not due to any
fundamental disagreements between developers. Everyone is keen to see them
merged.
	</para>
	<para>
This specification proposes:

		<itemizedlist>
			<listitem><para>
A standard way for applications to install new MIME related information.
			</para></listitem>
			<listitem><para>
A standard way of getting the MIME type for a file.
			</para></listitem>
			<listitem><para>
A standard way of getting information about a MIME type.
			</para></listitem>
			<listitem><para>
Standard locations for all the files, and methods of resolving conflicts.
			</para></listitem>
		</itemizedlist>
Further, the existing databases have been merged into a single package
<citation>SharedMIME</citation>.
	</para>
	<sect2 id="s2_layout">
		<title>Directory layout</title>
		<para>
There are two important requirements for the way the MIME database is stored:
			<itemizedlist>
				<listitem><para>
Applications must be able to extend the database in any way when they are installed,
to add both new rules for determining type, and new information about specific types.
				</para></listitem>
				<listitem><para>
It must be possible to install applications in /usr, /usr/local and the user's home directory
(in the normal Unix way) and have the MIME information used.
				</para></listitem>
			</itemizedlist>
		</para>
		<para>
The directories to be used to store the files in the database are:
			<itemizedlist>
				<listitem><para>
<filename>/usr/share/mime/</filename>
				</para></listitem>
				<listitem><para>
<filename>/usr/local/share/mime/</filename>
				</para></listitem>
				<listitem><para>
<filename>~/.mime/</filename>
				</para></listitem>
			</itemizedlist>
In the rest of this document, paths shown with the prefix
<filename>&lt;MIME&gt;</filename> indicate the files should be loaded from
all the directories listed above. For example, <quote>Load all the
<filename>&lt;MIME&gt;/text/html.xml</filename> files</quote> means to load
<filename>/usr/share/mime/text/html.xml</filename>,
<filename>/usr/local/share/mime/text/html.xml</filename>, and
<filename>~/.mime/text/html.xml</filename> (if they exist).
		</para>
		<para>
Each application that wishes to contribute to the MIME database will install a
single XML file, named after the application, into one of the three
<filename>&lt;MIME&gt;/packages/</filename> directories (depending on where the user requested
the application be installed). After installing, uninstalling or modifying this
file, the application MUST run the <command>update-mime-database</command> command,
which is provided by the freedesktop.org shared database<citation>SharedMIME</citation>.
		</para>
		<para>
<command>update-mime-database</command> is passed the <filename>mime</filename>
directory containing the <filename>packages</filename> subdirectory which was
modified as its only argument. It scans all the XML files in the <filename>packages</filename>
subdirectory, combines the information in them, and creates a number of output files.
		</para>
		<para>
Where the information from these files is conflicting, information from directories
lower in the list takes precedence.
Any file named <filename>Override.xml</filename> takes precedence over all other files in
the same <filename>packages</filename> directory. Tools which let the user edit the
database should edit the file <filename>~/.mime/packages/Override.xml</filename>.
		</para>
		<para>
The files created by <command>update-mime-database</command> are:
			<itemizedlist>
				<listitem><para>
<filename>&lt;MIME&gt;/globs</filename> (contains a mapping from extensions to MIME types)
				</para></listitem>
				<listitem><para>
<filename>&lt;MIME&gt;/magic</filename> (contains a mapping from file contents to MIME types)
				</para></listitem>
				<listitem><para>
<filename>&lt;MIME&gt;/XMLnamespaces</filename> (contains a mapping from XML
(namespaceURI, localName) pairs to MIME types)
				</para></listitem>
				<listitem><para>
<filename>&lt;MIME&gt;/MEDIA/SUBTYPE.xml</filename> (one file for each MIME
type, giving details about the type)
				</para></listitem>
			</itemizedlist>
The format of these generated files and the source files in <filename>packages</filename>
are explained in the following sections. This step serves several purposes. First, it allows
applications to quickly get the data they need without parsing all the source XML files (the
base package alone is over 700K). Second, it allows the database to be used for other
purposes (such as creating the <filename>/etc/mime.types</filename> file if
desired). Third, it allows validation to be performed on the input data,
and removes the need for other applications to carefully check the input for
errors themselves.
		</para>
	</sect2>
	<sect2>
		<title>The source XML files</title>
		<para>
Each application provides only a single XML source file, which is installed in the
<filename>packages</filename> directory as described above. This file is an XML file
whose document element is named <userinput>mime-info</userinput> and whose namespace URI
is <ulink url="http://www.freedesktop.org/standards/shared-mime-info"/>. All elements
described in this specification MUST have this namespace too.
		</para><para>
The document element may contain zero or more <userinput>mime-type</userinput> child nodes,
in any order, each describing a single MIME type. Each element has a <userinput>type</userinput>
attribute giving the MIME type that it describes.
		</para><para>
Each <userinput>mime-type</userinput> node may contain any combination of the following elements,
and in any order:
			<itemizedlist>
				<listitem><para>
<userinput>glob</userinput> elements have a <userinput>pattern</userinput> attribute. Any file
whose name matches this pattern will be given this MIME type (subject to conflicting rules in
other files, of course).
		</para>
		<para>
KDE's glob system replaces GNOME's and ROX's ext/regex fields, since it
is trivial to detect a pattern in the form '*.ext' and store it in an
extension hash table internally. The full power of regular expressions was
not being used by either desktop, and glob patterns are more suitable for
filename matching anyway.
				</para></listitem>
				<listitem><para>
<userinput>magic</userinput> elements contain a list of
<userinput>match</userinput> elements, any of which may match, and an optional
<userinput>priority</userinput> attribute for all of the contained rules. Low
numbers should be used for more generic types (such as 'gzip compressed data')
and higher values for specific subtypes (such as a word processor format that
happens to use gzip to compress the file). The default priority value is 50, and
the maximum is 100.
				</para><para>
Each <userinput>match</userinput> element has a number of attributes:

<informaltable>
	<tgroup cols="3">
	<thead><row><entry>Attribute</entry><entry>Required?</entry><entry>Value</entry></row></thead>
	<tbody>

	<row><entry>type</entry><entry>Yes</entry><entry>
<userinput>string</userinput>, <userinput>host16</userinput>,
<userinput>host32</userinput>, <userinput>big16</userinput>,
<userinput>big32</userinput>, <userinput>little16</userinput>,
<userinput>little32</userinput> or <userinput>byte</userinput>.
	</entry></row>

	<row><entry>offset</entry><entry>Yes</entry><entry>The byte offset(s)
	in the file to check. This may be a single number or a range in the
	form `start:end', indicating that all offsets in the range should be
	checked. The range is inclusive.</entry></row>

	<row><entry>value</entry><entry>Yes</entry><entry>
	The value to compare the file contents with, in the format indicated by the type
	attribute.
	</entry></row>

	<row><entry>mask</entry><entry>No</entry><entry>
	The number to AND the value in the file with before comparing it to
	`value'. Masks for numerical types can be any number, while masks for strings
	must be in base 16, and start with 0x.
	</entry></row>

	</tbody></tgroup>
</informaltable>

Each element corresponds to one line of
<citerefentry><refentrytitle>file</refentrytitle>
<manvolnum>1</manvolnum></citerefentry>'s <filename>magic.mime</filename> file.
They can be nested in the same way to provide the equivalent of continuation
lines.
				</para></listitem>
				<listitem><para>
<userinput>comment</userinput> elements give a human-readable textual description of the MIME
type. There may be many of these elements with different <userinput>xml:lang</userinput> attributes
to provide the text in multiple languages.
				</para></listitem>
				<listitem><para>
<userinput>root-XML</userinput> elements have <userinput>namespaceURI</userinput> 
and <userinput>localName</userinput> attributes. If a file is identified as being an XML file,
these rules allow a more specific MIME type to be chosen based on the namespace and localname
of the document element.
				</para><para>
If <userinput>localName</userinput> is present but empty then the document element may have
any name, but the namespace must still match.
				</para></listitem>
			</itemizedlist>
Applications may also define their own elements, provided they are namespaced to prevent collisions.
Unknown elements are copied directly to the output XML files like <userinput>comment</userinput>
elements. A typical use for this would be to indicate the default handler
application for a particular desktop
("Galeon is the GNOME default text/html browser"). Note that this doesn't
indicate the user's preferred application, only the (fixed) default.
		</para>
		<para>
Here is an example source file, named <filename>diff.xml</filename>:
		<programlisting><![CDATA[
<?xml version="1.0"?>
<mime-info xmlns='http://www.freedesktop.org/standards/shared-mime-info'>
  <mime-type type="text/x-diff">
    <comment>Differences between files</comment>
    <comment xml:lang="af">verskille tussen lêers</comment>
    ...
    <magic priority="50">
      <match type="string" offset="0" value="diff\t"/>
      <match type="string" offset="0" value="***\t"/>
      <match type="string" offset="0" value="Common subdirectories: "/>
    </magic>
    <glob pattern="*.diff"/>
    <glob pattern="*.patch"/>
  </mime-type>
</mime-info>
]]></programlisting>
		</para><para>
In practice, common types such as text/x-diff are provided by the freedesktop.org shared
database. Also, only new information needs to be provided, since this information will be merged
with other information about the same type.
		</para>
	</sect2>
	<sect2>
		<title>The MEDIA/SUBTYPE.xml files</title>
		<para>
These files have a <userinput>mime-type</userinput> element as the root node. The format is
as described above. They are created by merging all the <userinput>mime-type</userinput>
elements from the source files and creating one output file per MIME type. Each file may contain
information from multiple source files. The <userinput>magic</userinput> and
<userinput>glob</userinput> elements will have been removed.
		</para>
		<para>
The example source file given above would (on its own) create an output file called
<filename>&lt;MIME&gt;/text/x-diff.xml</filename> containing the following:
			<programlisting><![CDATA[
<?xml version="1.0" encoding="utf-8"?>
<mime-type xmlns="http://www.freedesktop.org/standards/shared-mime-info" type="text/x-diff">
<!--Created automatically by update-mime-database. DO NOT EDIT!-->
  <comment>Differences between files</comment>
  <comment lang="af">verskille tussen lêers</comment>
  ...
</mime-type>

]]></programlisting>
		</para>
	</sect2>
	<sect2>
		<title>The glob files</title>
		<para>
This is a simple list of lines containing a MIME type and pattern, separated by a colon.
For example:
			<programlisting><![CDATA[
# This file was automatically generated by the
# update-mime-database command. DO NOT EDIT!
...
text/x-diff:*.diff
text/x-diff:*.patch
...
]]></programlisting>
		</para>
		<para>
Applications MUST first try a case-sensitive match, then a case-insensitive
one. This is so that <filename>main.C</filename> will be seen as a C++ file,
but <filename>IMAGE.GIF</filename> will still use the *.gif pattern.
		</para>
		<para>
If several patterns match then the longest pattern SHOULD be used. In
particular, files with multiple extensions (such as
<filename>Data.tar.gz</filename>) MUST match the longest sequence of extensions
(eg '*.tar.gz' in preference to '*.gz'). Literal patterns (eg, 'Makefile') must
be matched before all others. It is acceptable to match patterns of the form
'*.text' before other wildcarded patterns (that is, to special-case extensions
using a hash table).
		</para>
		<para>
There may be several rules mapping to the same type. They should all be merged.
If the same pattern is defined twice, then they MUST be ordered by the
directory the rule came from, as described above.
		</para>
		<para>
Common types (such as MS Word Documents) will be provided in the X Desktop
Group's package, which MUST be required by all applications using this
specification. Since each application will then only be providing information
about its own types, conflicts should be rare.
		</para>
	</sect2>
	<sect2>
		<title>The magic files</title>
		<para>
The magic data is stored in a binary format for ease of parsing. The old magic database
had complex escaping rules; these are now handled by <command>update-mime-database</command>.
		</para><para>
The file starts with the magic string "MIME-Magic\0\n".
There is no version number in the file. Incompatible changes will be handled by
creating both the current `magic' file and a newer `magic2' in the new format.
Where possible, compatible changes only will be made.
All numbers are big-endian, so need to be byte-swapped on little-endian machines.
		</para><para>
The rest of the file is made up of a sequence of small sections.
Each section is introduced by giving the priority and type in brackets, followed by
a newline character. Higher priority entries come first. Example:
<screen>[50:text/x-diff]\n</screen>
Each line in the section takes the form:
<screen>[ indent ] ">" start-offset "=" value
[ "&amp;" mask ] [ "~" word-size ] [ "+" range-length ] "\n"</screen>
<informaltable>
	<tgroup cols="3">
	<thead><row><entry>Part</entry><entry>Example</entry><entry>Meaning</entry></row></thead>
	<tbody>

	<row><entry>indent</entry><entry>1</entry><entry>The nesting
	depth of the rule, corresponding to the number of '>' characters in the traditional file format.</entry></row>
	<row><entry>">" start-offset</entry><entry>&gt;4</entry><entry>The offset into the
	file to look for a match.</entry></row>
	<row><entry>"=" value</entry><entry>=\0x0\0x2\0x55\0x40</entry><entry>
	Two bytes giving the (big-endian) length of the value, followed by the value itself.
	</entry></row>
	<row><entry>"&amp;" mask</entry><entry>&amp;\0xff\0xf0</entry><entry>
	The mask, which (if present) is exactly the same length as the value.
	</entry></row>
	<row><entry>"~" word-size</entry><entry>~2</entry><entry>On little-endian machines, the
	size of each group to byte-swap.</entry></row>
	<row><entry>"+" range-length</entry><entry>+8</entry><entry>The length of the region
	in the file to check.
	</entry></row>
	</tbody>
	</tgroup>
</informaltable>
		</para><para>
Note that the value, value length and mask are all binary, whereas everything
else is textual. Each of the elements begins with a single character to
identify it, except for the indent level.
		</para><para>
The word size is used for byte-swapping. Little-endian systems should reverse
the order of groups of bytes in the value and mask if this is greater than one.
This only affects `host' matches (`big32' entries still have a word size of 1,
for example, because no swapping is necessary, whereas `host32' has a word size
of 4).
		</para><para>
The indent, range-length, word-size and mask components are optional. If
missing, indent defaults to 0, range-length to 1, the word-size to 1, and the
mask to all 'one' bits.
		</para><para>
Indent corresponds to the nesting depth of the rule. Top-level rules have an
indent of zero. The parent of an entry is the preceding entry with an indent
one less than the entry.
		</para><para>
If an unknown character is found where a newline is expected then the whole
line should be ignored (there will be no binary data after the new
character, so the next line starts after the next "\n" character). This is for
future extensions.
		</para><para>
The text/x-diff above example would (on its own) create this magic file:
			<programlisting><![CDATA[
00000000  4d 49 4d 45 2d 4d 61 67  69 63 00 0a 5b 35 30 3a  |MIME-Magic..[50:|
00000010  74 65 78 74 2f 78 2d 64  69 66 66 5d 0a 3e 30 3d  |text/x-diff].>0=|
00000020  00 05 64 69 66 66 09 0a  3e 30 3d 00 04 2a 2a 2a  |..diff..>0=..***|
00000030  09 0a 3e 30 3d 00 17 43  6f 6d 6d 6f 6e 20 73 75  |..>0=..Common su|
00000040  62 64 69 72 65 63 74 6f  72 69 65 73 3a 20 0a     |bdirectories: .|
]]></programlisting>
		</para>
	</sect2>
	<sect2>
		<title>The XMLnamespaces files</title>
		<para>
Each <filename>XMLnamespaces</filename> file is a list of lines in the form:
<screen>namespaceURI " " localName " " MIME-Type "\n"</screen>
For example:
<screen>
http://www.w3.org/1999/xhtml html application/xhtml+xml
</screen>
The lines are sorted (using strcmp) and there are no lines with the same namespaceURI and
localName in one file. If the localName was empty then there will be two spaces following
the namespaceURI.
		</para>
	</sect2>
	<sect2>
		<title>Storing the MIME type using Extended Attributes</title>
		<para>
An implementation MAY also get a file's MIME type from the
<userinput>user.mime_type</userinput> extended attribute. <!-- The attr(5) man
page documents this name --> The type given here should normally be used in
preference to any guessed type, since the user is able to set it explicitly.
Applications MAY choose to set the type when saving files. Since many
applications and filesystems do not support extended attributes,
implementations MUST NOT rely on this method being available.
		</para>
	</sect2>
	<sect2>
		<title>Recommended checking order</title>
		<para>
Because different applications have different requirements, they may choose to
use the various methods provided by this specification in any order. However, the
RECOMMENDED order to perform the checks is:
			<itemizedlist>
				<listitem><para>
If a MIME type is provided explicitly (eg, by a ContentType HTTP header, a MIME
email attachment, an extended attribute or some other means) then that should
be used instead of guessing.
				</para></listitem>
				<listitem><para>
If no explicit type is present, the glob rules should be applied to the name to
get the type.
				</para></listitem>
				<listitem><para>
If no glob rules match, the magic rules should be tried next.
				</para></listitem>
				<listitem><para>
If nothing matches, the default type of application/octet-stream should be used
for binary data, or text/plain for textual data (checking the start of the file
for ASCII control characters is a good way to guess whether a file is binary or
text).
				</para></listitem>
			</itemizedlist>
		</para>
		<para>
There are several reasons for checking the globs patterns before the magic.
Some applications don't check the magic at all, and this makes it more likely
that both will get the same type. Users can easily understand why calling their
text file <filename>README.mp3</filename> makes the system think it's an MP3,
whereas they have trouble understanding why their computer thinks
<filename>README.txt</filename> is a PostScript file. If the system guesses wrongly,
the user can often rename the file to fix the problem.
		</para>
	</sect2>
	<sect2>
		<title>Security implications</title>
		<para>
The system described in this document is intended to allow different programs
to see the same file as having the same type. This is to help interoperability.
The type determined in this way is only a guess, and an application MUST NOT
trust a file based simply on its MIME type. For example, a downloader should
not pass a file directly to a launcher application without confirmation simply
because the type looks `harmless' (eg, text/plain).
		</para>
		<para>
Do not rely on two applications getting the same type for the same file, even
if they both use this system. The spec allows some leeway in implementation,
and in any case the programs may be following different versions of the spec.
		</para>
	</sect2>
	<sect2>
		<title>User modification</title>
		<para>
The MIME database is NOT intended to store user preferences. Users should never
edit the database. If they wish to make corrections or provide MIME entries for
software that doesn't provide these itself, they should do so by means of the
Override.xml mentioned in <xref linkend="s2_layout"/>. Information such as
"text/html files need to be opened with Mozilla" should NOT go in the database.
		</para>
	</sect2>
</sect1>

<sect1>
	<title>Contributors</title>
	<simplelist>
		<member>
			Thomas Leonard <email>tal197@users.sf.net</email>
		</member>
		<member>
			David Faure <email>david@mandrakesoft.com</email>
		</member>
		<member>
			Alex Larsson <email>alexl@redhat.com</email>
		</member>
		<member>
			Seth Nickell <email>snickell@stanford.edu</email>
		</member>
		<member>
			Keith Packard <email>keithp@keithp.com</email>
		</member>
		<member>
			Filip Van Raemdonck <email>mechanix@debian.org</email>
		</member>
		<member>
			Christos Zoulas <email>christos@zoulas.com</email>
		</member>
	</simplelist>
</sect1>

<bibliography>
	<title>References</title>

	<bibliomixed>
		<abbrev>GNOME</abbrev><citetitle>The GNOME desktop,
		<ulink url="http://www.gnome.org"/></citetitle>
	</bibliomixed>
	<bibliomixed>
		<abbrev>KDE</abbrev><citetitle>The KDE desktop,
		<ulink url="http://www.kde.org"/></citetitle>
	</bibliomixed>
	<bibliomixed>
		<abbrev>ROX</abbrev><citetitle>The ROX desktop,
		<ulink url="http://rox.sourceforge.net"/></citetitle>
	</bibliomixed>
	<bibliomixed>
		<abbrev>DesktopEntries</abbrev><citetitle>Desktop Entry Specification,
		<ulink url="http://www.freedesktop.org/standards/desktop-entry-spec.html"/>
		</citetitle>
	</bibliomixed>
	<bibliomixed>
		<abbrev>SharedMIME</abbrev><citetitle>Shared MIME-info Database,
		<ulink url="http://www.freedesktop.org/standards/shared-mime-info.html"/>
		</citetitle>
	</bibliomixed>
	<bibliomixed>
		<abbrev>RFC-2119</abbrev>
		<citetitle>Key words for use in RFCs to Indicate Requirement Levels,
		<ulink url="http://www.ietf.org/rfc/rfc2119.txt?number=2119"/>
		</citetitle>
	</bibliomixed>
</bibliography>

</article>