[Mimedefang] No boilerplate on some messages

[Dave Williss]

I've noticed that messages which are of type "text/html" (only, no
multipart) don't
get the $Boilerplate added anywhere.

I've also tried adding the message using $entity->attach() (and entity is
valid
in the place were I do it), but some messages get the attachment and some
don't.

A quick check shows that most of the ones where the attach doesn't work are
also of type "text/html" or "multipart/mixed".

(Ick!  The multipart/mixed one only has one part and it's html)

 -- Dave Williss
------
Meddle not in the affairs of dragons,
   for you are crunchy and taste good with catsup