Added default_content_type argument to

Added default_content_type argument to
make_mime_composition_file_entry().  It's used only to force message
bodies to always use text/plain content type, which allows mhbuild to
do the right thing with them.  Thanks to Steve Rader for reporting
that UTF-8 characters in message bodies had previously caused them to
be encoded as octet-streams (with attachments managed by send), and to
Ken for coming up with the fix.