I'm afraid might be in a dead end here. I don't think the server can request just any file from the client, that would be like a MAJOR security breach (f.ex. every server would have a file upload, which would discard the name and upload /etc/passwd or the Windows registry or something).
I think the uploaded file's name is in the request that the client sends, and if I remember correctly, the uploaded file comes next. I don't think the webserver is even allowed to say anything before the entire file is uploaded, and therefore you can't request a file that way, but I'm on very thin ice here. Anyway, it sounds very dangerous to me.