unfortunately to go further I'd need to see the FULL message. ALL headers from top to bottom. First character of message to last. Your email address and some IP addresses will be in there, you can sub them if you want to but make sure to keep them clear as to their purpose. (change ip addresses to 256.256.256.256 for example, instead of deleting the line it's on)
I'd venture a guess that there's something somewhat nonstandard or unusual about the attachment format that's confusing mail.app
