Content filtering leaves a single sub-part in a multipart message.

[Mark Sapiro]

If the original message is multipart but not multipart/alternative and content filtering removes all but one part, the message is left as a multipart message with only the one sub-part instead of being recast as just the sub-part.

This can cause msg_header and/or msg_footer to be added as separate parts to a text/plain message instead of being added to the text/plain body part.