Sending file content using urlmultipartbypost method in Oracle CPQ

Content

I am working on integration in which I have to send proposal document from CPQ to external system. It accepts Content Type: multipart/form-data. 

How do I send the file content from CPQ to a system that accepts Content Type: multipart/form-data. 

I am getting below error when tried sending the document using the script mentioned below: 

{"type":"validation_error","detail":"There is no field: \"file\"."},

Code Snippet

	ret = "";
         d = dict("dict<anytype>");
         put(d, PDFDocument_t, getattachmentdata(PDFDocument_t, true));
         params =  "{\"name\": \"Sample Document\"}";
         headers = dict("string");
         put(headers, "Content-Type", "multipart/form-data");
         put(headers,"Authorization", authdetails);
         soapResponse =urlmultipartbypost(url, params, headers,d);
         print soapResponse;
return ret;