Add image/x-kiss-cel
GIMP supports this image file format. "Cel" is short for "celluloid" and is not a typo.
The test suite already had a KiSS cel file and says that it is supposed to be image/x-cel, but I could not find any examples of anyone using image/x-cel in practice. On the other hand, I did find an example of image/x-kiss-cel being used and it's a better name anyway because the .cel file extension is also used for Autodesk Animator cel image files and Lumena "cell outline" images.
https://gitlab.gnome.org/GNOME/gimp/-/blob/GIMP_2_10_38/plug-ins/common/file-cel.c https://en.wikipedia.org/wiki/Cel https://snisurset.net/code/abydos/formats.html http://justsolve.archiveteam.org/wiki/KiSS_CEL http://justsolve.archiveteam.org/wiki/Animator_PIC/CEL http://justsolve.archiveteam.org/wiki/Lumena_CEL