Posting multipart XML data to rails

I'm posting xml data to rails. I'd like to be able to include a file within the request, probably by sending both xml & binary data as a single multipart request.

As far as I can tell, Rails' current param_parsers methods won't handle this. I confess I'm struggling to even get ruby's CGI class to handle multipart data... has anyone else succeeded in writing their own param_parser that will handle multipart XML? Am I missing any alternative methods?