Displaying BMP Images in Emacs 27.1's Image Major Mode.

[Ralph Corderoy]

Hi,

On Debian 11, Bullseye, I've package emacs 1:27.1+1-3.1 installed on a
remote server.  If I ‘ssh -Y’ to the machine and run ‘emacs
/tmp/foo.png’ then the X Windows version of Emacs appears on the local X
server with the top-half of the window showing foo.png as pixels in
‘(Image[png])’ mode.  All is well.

Note, I have no ~/.emacs file as the remote user.

But if I run ‘emacs /tmp/foo.bmp’ instead then I see the characters of
the file in the top-half buffer and it's in ‘(Fundamental Image)’ mode.
At the bottom of the window it says ‘Cannot display image: (Invalid
image type `bmp')’.  ^C ^X switches to the hex-dump and ^C ^C switches
back, as I'd expect in Image mode.

I'm aware BMP needs an external converter.  The remote machine has
Debian packages for graphicsmagick and imagemagick-6.q16 installed.
gm(1) works fine, for example.

I used customize to set image-use-external-converter to t but that
didn't improve the above BMP outcome after quitting and re-running.
The setting did appear in a new ~/.emacs.

I've also replaced ~/.emacs with

    (auto-image-file-mode 1)
    (setq image-use-external-converter t
          image-converter 'graphicsmagick)

based on some ’net searching with no improvement.

Using ‘strace -fe execve emacs /tmp/foo.bmp’, I see no attempt to run
gm(1) or similar to learn what formats they support or try a conversion.

Please put me out of my misery or, if you think that's too extreme,
let me know few scant lines in ~/.emacs would display the BMP.

-- 
Thanks, Ralph.