My problem is simple. I need to upload a file directly to the correct server (which has currently low workload).
Therefore I do:
You could try returning a code 302 (temporary moved), not 100% that would let your browser post the data to the changed url though, but it's worth something to check out.
[Edit]
According to this wikipedia article, the POST data would be converted to GET, which probably won't work for a file upload.