Add content type detection and inclusion.

This is necessary, for instance, for mp3 files to play properly in Firefox in
the html5 native player.  It will not play audio if the content type is
binary/octet-stream (which everything is defaulting to currently)

Also though it's probably a more like a bug than a feature, we should do a minor version bump if we merge this. Changing the content_type on the uploaded files is a pretty non-trivial thing to start happening on an asset upload gem.