thanks for the quick response!
i think i got the enctype right, but here's the form header just in case :
i normally use Opera but i tested it in IE as well and it gave the same error in both browsers.
<form method="post" action="cgi-bin/formmail.php" enctype="multipart/form-data" name="Request_Form" />
so it could be something wrong on the server side.
i did however try uploading a file using the file upload interface in my server's webmin panel, and that worked fine - don't know if that's at all relevant though
edit : forgot to add, i was testing the upload with a PDF file, if that makes any difference. the server is redhat linux and i think the it's running php4.something.