I can't reproduce this either, and without a reproducer we might as well close it.
Antoine it is possible that your fix for #5853 inadvertently fixed this, but I don't feel like untangling the logic of the module enough to figure it out :) So I'm going to close it 'works for me'. If S Arrowsmith can reproduce it with 2.7r2, we can reopen.
By the way, it produced 'jpe' for me, too...but, then, my system (Gentoo) /etc/mime.types file has 'jpe' as the first filetype for jpeg, so I don't think that association is Python's bug, per se. Though I may eventually have to address it in email6. (Also by the way, I tried switching the order and passing in the modified file explicitly on the explicit init, but that didn't change the behavior).