EMailParserMessage: Better handle message bodies as attachments.

Simo Sorce sent me an interesting case where the MIME type of the
message itself was image/gif, but the image was not being shown.

If the EMailPart representing the message body is marked as an
attachment, wrap it as such so it gets added to the attachment
bar but also set the "force_inline" flag since it doesn't make
sense to collapse the message body if we can render it. */

(cherry picked from commit 514736f2)