Re: image.el doesn't associate image-mode with .JPG files

[Jason Rumney]

Mathias Dahl wrote:
> Richard Stallman <address@hidden> writes:
>
>   
> >     If .JPG/.JPEG is frequent, perhaps we should add it to
> >     `auto-mode-alist', then. Or do this:
> >
> >      (push '("^\xFF\xD8\xFF\xE0\x00\x10JFIF" . image-mode)
> >            magic-mode-alist)
> >
> > I am not sure which is better, but I agree we should do one or the
> > other.
> >     
>
> I guess it is a matter of taste, but I would prefer if Emacs uses the
> technique above instead of looking at the file name.
>   

The only problem with this method is that the actual regexp you need is 
more complex, as there are different versions of JFIF, and Exif to check 
for (most digital cameras use the latter). image-jpeg-p has a much more 
relaxed check, basically "^\xFF\xD8\xFF[\xE0-\xEF]..+(JFIF|Exif)", but 
with the search for JFIF|Exif limited based on the first two bytes of 
the ..+ portion - which indicate the length of the file header. If you 
can't limit the search to the header, then the regexp may become too 
inefficient for use in magic-mode-alist.