Re: C file recoginzed as image file

[Chris Moore <dooglus@gmail.com>]

Richard Stallman <rms@gnu.org> writes:

>     Similarly, any text file which starts with the 4 characters "GIF8" is
>     treated as if it was an image.
>
> Is there anything else we could check for
> to distinguish real GIF files from other files?
> Would someone please check the spec?

http://www.w3.org/Graphics/GIF/spec-gif89a.txt tells me that the first
3 bytes contain the fixed value 'GIF', and the next 3 bytes contain
the version number:

            Version Numbers as of 10 July 1990 :       "87a" - May 1987
                                                       "89a" - July 1989

            Version numbers are ordered numerically increasing on the first two
            digits starting with 87 (87,88,...,99,00,...,85,86) and
            alphabetically increasing on the third character (a,...,z).

Immediately after this 6 byte header comes the logical screen
descriptor, which doesn't contain anything we can use to recognise a
valid image.

So a better image-type-header-regexps entry would be:

    ("\\`GIF[0-9][0-9][a-z]" . gif)