Part 1 has to be the message. Part 2 must be a PGP/MIME signature. This should be better.
Furthermore, the error messages that currently exist could be improved:
22:08:57 < radix> Odd_Bloke: does it require use of PGP/MIME?
22:09:58 -!- oleavr [<email address hidden>] has quit []
22:10:55 < radix> odd_bloke: the reason I ask is that I got this error: PQMException: 'Second part of multipart message must be application/pgp-signature'
22:11:04 < Odd_Bloke> radix: Nope, inline is fine.
22:11:25 < radix> hmm, then what's with the error?
22:11:36 < Odd_Bloke> radix: You can only send multipart messages if the first part is the content and the second part is application/pgp-signature.
22:11:42 < Odd_Bloke> And even that's not recommended.
22:11:49 < radix> oh, so if my message happens to be multipart it will assume it's going to be PGP/MIME?
22:11:59 < Odd_Bloke> Yeah, afraid so.
22:12:03 < radix> Hmm, I don't think I *tried* to send a multipart message, but who knows what evolution does
For one, this means that you can't use "bzr send" when your mail_client is set to "editor", because bzr sends those messages with the merge directive as an attachment.