I'd venture a guess that there's something somewhat nonstandard or unusual about the attachment format that's confusing mail.app
From past experience I would not be surprised if the non-standard bit was an embedded non-ascii character that is tricking the MIME decoder. I ended up finding the character with a hex editor run on the raw text file. The character was apparently being injected at the IMAP server. The ISP updated their server software and the problem never recurred.
