Forwarding imap messages as attachments doesn't include forwarded message's body - this is a regression from the filespec removal, I'm pretty sure. Forwarding local messages seems to be fine.
Actually, this affects local messages as well - the actual bug is that the first couple hundred bytes of the message body seems to be getting skipped...very strange.
The bug is here: http://lxr.mozilla.org/mozilla/source/mailnews/compose/src/nsMsgSendPart.cpp#729 ReadLine reads ahead into a buffer, which throws off the file pointer. So we need to either seek back to where the previous line ended, or somehow figure out how to get the data out of the line buffer. Or, most likely, just stop using the lineinput stream, and use NS_ReadLine directly.
Created attachment 262917 [details] [diff] [review] proposed fix use NS_ReadLine, which lets us know how far off the underlying stream pointer is...