As far as I know, no one has written code for Arc yet to decode multipart/form-data.
I was too lazy to work on a multipart/form-data decoder myself... since I run a number of servers, it was easy enough for me to have the form posted to another one of my servers (Perl in my case) to handle the file upload, and then I have it redirect the user back to my Arc server to continue after the file upload.
