Do not consider calender-invitations "attachments"; do mark as
"calendar". Do recognize application/ics as calendar messages.
Update mime-object to expose a message part's disposition.
Change the "is-attachment" heuristic to include inline parts if they
have a filename parameter in their content-disposition.
Note that this doesn't change things radically; the delta is +69 and
-202 for ~6500 attachments.
Supported a "cooked" mode for attachment filenames, which gets rid of any
unacceptable characters. Add "raw_filename" to get the filename as specified in
the part.
Update tests.
