Default exotic image formats (like .webp) to image-mode

* doc/lispref/errors.texi (Standard Errors): Mention the new error.

* lisp/files.el (auto-mode-alist): Add a bunch of image suffixes
to the list (bug#37972) based on the output from "gm convert -list
format" (i.e., graphicsmagick).

* lisp/image-mode.el (image-mode): Rewrite to possibly notify the
user about image-use-external-converter.
(image-mode--setup-mode): Factor out into own function and don't
run under `condition-case' as there's nothing here that should
error.

* lisp/image.el (unknown-image-type): New error.
(image-type): Signal that error so that image-mode can offer
sensible feedback to the user.

1 files changed, 45 insertions, 1 deletions

diff --git a/lisp/files.el b/lisp/files.el
index 4b364b49d84..f6dc4baa7fb 100644
--- a/lisp/files.el
+++ b/lisp/files.el
@@ -2847,7 +2847,51 @@ ARC\\|ZIP\\|LZH\\|LHA\\|ZOO\\|[JEW]AR\\|XPI\\|RAR\\|CBR\\|7Z\\)\\'" . archive-mo
      ;; The following should come after the ChangeLog pattern
      ;; for the sake of ChangeLog.1, etc.
      ;; and after the .scm.[0-9] and CVS' <file>.<rev> patterns too.
-     ("\\.[1-9]\\'" . nroff-mode)))
+     ("\\.[1-9]\\'" . nroff-mode)
+     ;; Image file types probably supported by `image-convert'.
+     ("\\.art\\'" . image-mode)
+     ("\\.avs\\'" . image-mode)
+     ("\\.bmp\\'" . image-mode)
+     ("\\.cmyk\\'" . image-mode)
+     ("\\.cmyka\\'" . image-mode)
+     ("\\.crw\\'" . image-mode)
+     ("\\.dcr\\'" . image-mode)
+     ("\\.dcx\\'" . image-mode)
+     ("\\.dng\\'" . image-mode)
+     ("\\.dpx\\'" . image-mode)
+     ("\\.fax\\'" . image-mode)
+     ("\\.hrz\\'" . image-mode)
+     ("\\.icb\\'" . image-mode)
+     ("\\.icc\\'" . image-mode)
+     ("\\.icm\\'" . image-mode)
+     ("\\.ico\\'" . image-mode)
+     ("\\.icon\\'" . image-mode)
+     ("\\.jbg\\'" . image-mode)
+     ("\\.jbig\\'" . image-mode)
+     ("\\.jng\\'" . image-mode)
+     ("\\.jnx\\'" . image-mode)
+     ("\\.miff\\'" . image-mode)
+     ("\\.mng\\'" . image-mode)
+     ("\\.mvg\\'" . image-mode)
+     ("\\.otb\\'" . image-mode)
+     ("\\.p7\\'" . image-mode)
+     ("\\.pcx\\'" . image-mode)
+     ("\\.pdb\\'" . image-mode)
+     ("\\.pfa\\'" . image-mode)
+     ("\\.pfb\\'" . image-mode)
+     ("\\.picon\\'" . image-mode)
+     ("\\.pict\\'" . image-mode)
+     ("\\.rgb\\'" . image-mode)
+     ("\\.rgba\\'" . image-mode)
+     ("\\.tga\\'" . image-mode)
+     ("\\.wbmp\\'" . image-mode)
+     ("\\.webp\\'" . image-mode)
+     ("\\.wmf\\'" . image-mode)
+     ("\\.wpg\\'" . image-mode)
+     ("\\.xcf\\'" . image-mode)
+     ("\\.xmp\\'" . image-mode)
+     ("\\.xwd\\'" . image-mode)
+     ("\\.yuv\\'" . image-mode)))
   "Alist of filename patterns vs corresponding major mode functions.
 Each element looks like (REGEXP . FUNCTION) or (REGEXP FUNCTION NON-NIL).
 \(NON-NIL stands for anything that is not nil; the value does not matter.)