http://arclanguage.com/item?id=10462
It overwrites the built in functions to parse out the form parts, but it only works on *nix systems. This was a while ago and solved my problem. ymmv ;)
http://github.com/coconutrandom/arc/blob/master/parseform.ar...
http://github.com/coconutrandom/arc/blob/master/upload-3.arc