Sending file via PostURL

I want to postURL data that includes a file that has been uploaded to a record. I created a simple function which should do it but I get a server error from the receiving web service. Probably because Zoho Creator automatically uses a Content-Type of application/x-www-form-urlencoded when what I really need to send is probably a different content-type (multipart/form-data?) to the url. Any ideas on how I can post a file using postURL?