XML + attachment(s) using AS2

Does anyone have experience using a multipart payload using AS2? I would like to be able to process signed multipart messages using AS2, so the MIME message might look like:

-multipart/related (AS2 header)
-application/XML } multipart payload
-image/GIF }

From the WM EDIINT documentation, the wm.EDIINT.receive service will re-submit the payload to TN after processing the AS2 envelope/signature. So what do I need to do to process a payload with more than one part?