from http://bugs.musicbrainz.org/ticket/2700

The default for the "Move additional files" option is "*.jpg *.png", however, since Linux is case sensitive, this will not move files saved as *.JPG or *.PNG. Suggest either making this not case sensitive or changing the default to include both.

I haven't tested this myself, but bitmap says it's definitely still a problem, judging by his settings.