In any 3.13.X version, the generation of thumbnails for gif images extracted from email does not work properly. The thumbnail appears as an all black image, but clicking on the link shows the gif file correctly. Strangely enough, this problem seems intermittent in 3.13.5, because in that version, the thumbnail is generated correctly sometimes.

This seems like a regression from 3.12.3, where the thumbnails are shown correctly.