db.fs.chunks.drop();
db.fs.chunks.insert({files_id:1,n:0,data:new BinData(0,"test")})

x = db.runCommand({"filemd5":1,"root":"fs"});
assert( ! x.ok , tojson(x) )

db.fs.chunks.ensureIndex({files_id:1,n:1})
x = db.runCommand({"filemd5":1,"root":"fs"});
assert( x.ok , tojson(x) )