Mails with PDF attachments sent with Mailer causes problems in lots of mail clients. It seems to be due to the "multipart/related" header. Changing to "multipart/mixed" makes the problem go away. Maybe we need a way for clients to control this header and while at it it would be nice to do real mixed content (inline attachments). At the moment Mailer puts all attachments at the end but "inline".
Imported from Assembla: http://www.assembla.com/spaces/liftweb/tickets/1197
Also see #1403, which I believe is related.