First check your message size limits. The default is 10MB. And look in /opt/zimbra/log/mailbox.log for more specific errors.
As of 5.0.2, I don't think convertd failures are supposed to break IMAP upload anymore. Anyway, I'm pretty sure this is available in the GUI (look in preferences or features?) but you can do it per CoS or per user at the command line, too.
zmprov mc Default zimbraAttachmentsIndexingEnabled FALSE
zmprov ma
foo@example.com zimbraAttachmentsIndexingEnabled FALSE