After a bit more research, it seems like Django uses [mimetypes.guess_type](http://www.python.org/doc//current/library/mimetypes.html) which should be able to detect common media types. It seems like for most cases, we can safely omit the ``mimetype`` argument.

In the future, if we want to support user specified mimetypes, we can perhaps mimic Django's current API of using triples (``(filename, content, mimetype)``)to specify attachments. What do you think @yprez?