Marc,
I've git bisected as far as 2010, and I can't seem to find a good commit. My pma_column_info table has: mimetype: image_jpeg transformation: image_jpeg__inline.inc.php and my local image is /home/alex/Pictures/x.jpeg, which has a good format.
Could you possibly confirm that this was working before merging my branch? Or could anyone provide a good image and commit where this transformation worked? Maybe my local configuration is faulty, I'll try an older version on some other computer.
-- Alex