The module is very simple and minimal. It consists of a small initialisation routine which maps file endings to mime types, a routine called "guess" which guesses the mime type, and extension to file name data.
A bug in the initialisation routine means it overwrites its own values, so .jpeg turns into image/pjpeg. I reported it here:
I'm not sure why the author created this module. MIME::Types was already mature when this module was first released in 2012. In my test, MIME::Types gets 13/16 file names to mime types correct, but MIME::Type::FileName gets only 8/16 right.
MIME::Type::FileName is simple and self-contained, but not very accurate.