Good question: I'd really be interested to know if you come across a solution. I took a quick browse through the Content Pack and it doesn't seem that it is something that is natively doable. I'll check with my OO developer wot knows SM pretty well on Monday and see if she can shed some light.
From my really rudimentary knowledge of SM, it's likely to be a reference to an object on a filesystem somewhere, so may be retrievable via a WSDL call.
My customer asked me just yesterday the same question. They are using HPSM v9.30. I tried using a custom WSDL we've used for years querying information on Line Items. That WSDL query response includes an attachment result field but it doesn't have anything in it even for a test LI with an attachment.
I found this document for v9.41 which sorta makes me think it might be possible - but not for my customer that is 🙂
If you are at v9.41 or above, it might be able to get it to work. That PDF makes me think that for v9.41 the attachment is included in the XML response back from SM as a encoded blob.