Multipart REST post, XML body with file

Hi,

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.

Etienne