Strip the mailbox header when uploading messages from files
Stuff which was created by `git format-patch` contains this header.
Dovecot deals with it reasonably well, but things start going south when
we rely on a correct format of such a message, and construct another one
by prepending a set of Resent-* headers to that. Suddenly, this stray
mailbox header is placed in the middle of headers and it breaks stuff.
TL;DR: try to accept garbage which user tries to upload. That 80
characters is in place as a desperate safeguard against randomly
stripping too much crap.
Change-Id: I48131ff1c308ce64fdb81d5a7d276f1b2439f8e5