There are several tickets that appear to be related, all related to MIME messages that have attachments but no body (or bodies in alternative content types)
The changes made by robin in SVN revision 413 have helped quite a bit, but there are still some quirks.
Most of these can probably be closed with a little bit of testing to be sure they have been fixed. If not, it would be nice if someone could take a closer look at them.
Ticket #1484050 (Bugs) Messages with attachment only (no body part) are not parsed correctly "attachment" (really the only part) shows up now and I can download it, but the message list does not indicate that an attachment is present in the message.
Ticket #1483818 (Feature Requests) Support for viewing Content-Type: application/pgp et al. I can't reproduce this, but it looks like the programs I'm using to test are encoding the messages differently than the messages the original ticket opener sees. Thunderbird/Enigmail-generated messages work OK for me. I opened a bunch of signed PGP/MIME messages from some mailing lists, all worked OK.
Ticket #1443276 (Bugs) mail rendering problem with base64 encoded messages This may be fixed now but oddly enough I can't find a test message with a base64-encoded text body to see if it works. base64-encoded attachment bodies work for me since SVN rev 413.
Ticket #1484072 (Bugs) Weird forward behaviour. When attempting to forward one of these messages with only an attachment and no body text, it is not included.
I'm not sure this one is really related or not, but it could use a couple people to test on their systems and comment on it or close it.
Ticket #1483968 (Bugs) Message is not showing content Messages from Cron show up fine for me on FreeBSD, but if someone on Linux could double check this can probably be closed.