Make sure we serve the right MIME type for file types the UI depends on, no matter what the Python mimetypes module (which we now only use as a fallback for performance reasons) thinks.
Make sure we serve the right MIME type for file types the UI depends on,
no matter what the Python mimetypes module (which we now only use as a
fallback for performance reasons) thinks.

This should fix issue #370.

jparyani/Mailpile,jparyani/Mailpile,laborautonomo/Mailpile,laborautonomo/Mailpile,laborautonomo/Mailpile,jparyani/Mailpile