Email::MIME never supported this properly, but there's real
instances of forwarded messages as message/rfc822 attachments.
message/news is legacy thing which we'll see in archives, and
message/global appears to be the new thing.
gmime also supports message/rfc2822, so we'll support it anyways
despite lacking other evidence of its existence.
Existing attachments remain downloadable as a whole message,
but individual attachments of subparts are now downloadable
and can be displayed in HTML, too.
Furthermore, ensure Xapian can now search for common headers
inside those messages as well as the message bodies.