Force multi-part?

I have a form of POST parameters and a funky URL like:

http://foo.com/bar?12345

Which, when requested just like that, end up being a POST. But the form needs to post as a multi-part. If I add a GET parameter to the list of params it does multi-part, but the site doesn't like that extra parameter. I tried adding a file upload as a parameter and screen-scraper just spins and spins and spins..

Any thoughts? Is there a way I can use POST parameters and force it to go with multi-part instead of just POST?

Admittedly, the site I'm scraping is pretty convoluted and picky.

R