Up to debian buster, feh was able to open the thumbnails inside my .cr2 files using Imlib2 directly; this has stopped working in debian bullseye, but I've found that there is a way to open them properly using dcraw: it only requires adding --conversion-timeout 5 (or any other suitable positive number) to the command line to enable the use of external programs.

so

feh 141140-img_5195.cr2

results into various errors including feh WARNING: 141140-img_5195.cr2 - No Imlib2 loader for that file format

while

feh9 --conversion-timeout 5 141140-img_5195.cr2

prints 141140-img_5195.cr2 is a Canon EOS 1100D image. and shows you the preview you wanted :)

And this shaves the neck of the yak, now I can proceed with the original task...