Thanks Boemien, I'll give it a try. For what it's worth, all the files for the album "2/3 (remixed)" are mp3 files.

Is there a way to see/know how the cover art extractor chooses the filename of the jpeg it saves to ~/.cache/media-art ? I could understand that it ignores the "/" for making the hash, but not why it would ignore the whole "(remixed)" part.