I’ve never tied adding an Attachment in BizTalk.
I think your best bet is to use helper .net component. I think you just need to add another part to you message and is the attachment. There was a really good sample about this on Scott Woodgates blog a few years ago now… I think it was here: http://blogs.msdn.com/scottwoo/archive/2004/04/19/116298.aspx
I’m not sure if the samples are still available. It added an infopath form as an Attachment – but the concept is the same.
Here is a links that might help too:
http://geekswithblogs.net/erwinai/archive/2006/06/20/82474.aspx
Hope this helps.