Here's a wacky message structure my mom sent me (using Apple Mail):
I 1 <no description> [multipa/alterna, 7bit, 653K]
I 2 |-><no description> [text/plain, utf-8, 2.0K]
I 3 `-><no description> [multipa/mixed, 7bit, 651K]
I 4 |-><no description> [text/html, quoted, windows-1252, 3.0K]
I 5 |->Typeface Ideas.pdf [applica/pdf ...]
I 6 `-><no description> [text/html, 7bit, us-ascii, 0.2K]
I haven't set up attachment detection properly, so %X says this message has
0 attachments.
Details of how this works and why it's hard are here:
http://www.mutt.org/doc/devel/manual.html#attachments