Attachments missing from messages in "inbox" folder in apple mail, when using google apps
I had a similar problem in Mail on Mountain Lion, where attachments did not show up for a message. When looking at the raw source (⌘⎇U) of the message, I also found that headers of the message were missing.

I managed to fix the issue by selecting the mailbox containing the broken email, and then using the "Rebuild" command from the "Mailbox" menu. After a short time, my emails reappeared, complete with attachments.