Multipart REST post, XML body with file


Can anyone shed light on how to do a classic RAILS REST POST (i.e.
post an XML structure representing an ActiveResource) ... whilst at
the same time posting a file?

My current code (working) is below. It does not have a file associated
with the post.

I know I have to do a multipart, but what I cant work out is that in a
multipart, each value has a key ... what would be the key for the main
XML body? Because, see, in the example below, the XML _is_ the body
... it has no key associated with it.