Sounds like a good idea. Unfortunately, I couldn’t get it to work.
In fact, I couldn’t even get the pdf thumbnailer example they give there to work, even by pasting it 1 to 1 into a .thumbnailer file. Instructions unclear? Or simply obsolete information? Probably the latter, since trying to get a thumbnail by hand by running the convert command with according options failed with errors. I also find it unclear as to what the %s (the size of the image) argument will be. Will it be one number (12), multiple numbers (12 12), or a string (“12px”)? I wasn’t able to dig deep enough to capture it for a closer look.
Anyway, back to the topic. I think in theory your suggestion should work like a charm. But unfortunately, the following thumbnailer I called
ffmpeg-audio-thumbnailer.thumbnailer causes no results:
Exec=ffmpeg -i %i %o
I was just testing it for mp3 files at first. Already to no avail. Yes, I did restart the system so changes would definitely take effect.
In the end, I found a solution by taking a read here: https://wiki.archlinux.org/index.php/File_manager_functionality#Thumbnail_previews
I simply installed
totem, GNOME’s default video player. The cover art for both mp3 and m4a files is now shown as expected. Though I did have to change my default application for opening music files back to VLC media player for both files types manually (right click -> file properties).
All in all, I don’t find this to be a very clean solution and more of a workaround. Getting things to work with a lightweight thumbnailer (without installing a bloated video player) would still be preferrable to me.