How to send file attachments to a Rails XML api ( from a .net cellphone application)

I have a Rails application setup to receive file attachments using
Paperclip (working fine on the web app side).

Now I need to allow a .net/C# cell phone application to post files to
the API - currently they can do a PUT request of XML and update the
record just fine, but not sure how to tell them to send the files -- I
don’t know if they should be doing a "multipart" thing like you do
with an HTML form or what. Initially they tried doing a base64
encoding of the image data and sending that in the <image> tag which
is the name of the file field that I have in the HTML form, but that
did not work. I would like to leave my web application as is, and just
find a way for them to adapt, but obviously I can switch to another
technology if need be, eg attachment_fu and a BLOB field, or do I need
to write some hook that decodes the base64 data back into a file and
then saves it?

I have found nothing in the way of documentation (I’ve been looking at
documentation with regards to CURL, Rails APIs, multipart XML, XML
with file attachments, etc etc)

Wondering if anybody has experience or advice?