You must use MIME over secure http POST to send attachments...
The http header of your multi-part MIME payload (set via commands you us to open the port) should include the following:
Content-type: multipart/related; boundary=something unique; type="text/xml"; start=<uniqueCID>
for example, the header would contain:
Content-type: multipart/related; boundary=abc;type="text/xml"; start=<uniqueCID>
Depending on the language/library (java/apache shown here) you use to create this transaction, the setting of these headers will change, but should look something like:
where payload is defined as an open https connection to the Ariba Network.
then the http payload would contain (important that it starts with the boundary you defined):
<?xml version="1.0" encoding="UTF-8"?>
This is described in cXML User Guide, Chapter 2, under the topic 'Wrapping Layers'