It turns out that solving this header problem via MIMEPart isn't really a good idea, given the rest of the API.  I don't want to not use 'type', or rather I do want to use the new policy.message_factory, and I don't really want to have to have two message_factory attributes.  I'm considering the possibility of only adding a MIME header in the Generator at the top level if and only if there are no parser derived headers.  (This will do the right think only if you stick to the new API, since the old API adds the header indiscriminately in several places).