UnityWebRequest ContentType not Overriding

The following code has worked for me to craft an HTTPWebRequest as a multipart/form-data request. Copying for ease of use (source at the bottom), I have also added some comments to make the code as clear as possible.

 ///<summary>Uploads form fields, and n files to REST endpoint</summary>
 ///<param name="url">URL to upload data to</param>
 ///<param name="files">Array containing string paths to files to upload</param>
 ///<param name="formFIelds">NameValueCollection containing all non-file fields from the form, default is null if no other fields provided</param>
 public static string UploadFilesToRemoteUrl(string url, string[] files, NameValueCollection formFields = null)
 {
     string boundary = "----------------------------" + DateTime.Now.Ticks.ToString("x");
 
     HttpWebRequest request = (HttpWebRequest) WebRequest.Create(url);
     request.ContentType = "multipart/form-data; boundary=" +
                             boundary;
     request.Method = "POST";
     request.KeepAlive = true;
     //stream contains request as you build it
     Stream memStream = new System.IO.MemoryStream();
   
     var boundarybytes = System.Text.Encoding.ASCII.GetBytes("\r\n--" +
                                                             boundary + "\r\n");
     var endBoundaryBytes = System.Text.Encoding.ASCII.GetBytes("\r\n--" +
                                                                 boundary + "--");
 
     //this is a multipart/form-data template for all non-file fields in your form data
     string formdataTemplate = "\r\n--" + boundary +
                                 "\r\nContent-Disposition: form-data; name=\"{0}\";\r\n\r\n{1}";
 
     if (formFields != null)
     {
         //utilizing the template, write each field to the request, convert this data to bytes, and store temporarily in memStream
         foreach (string key in formFields.Keys)
         {
             string formitem = string.Format(formdataTemplate, key, formFields[key]);
             byte[] formitembytes = System.Text.Encoding.UTF8.GetBytes(formitem);
             memStream.Write(formitembytes, 0, formitembytes.Length);
         }
     }
       //this is a multipart/form-data template for all fields containing files in your form data
     string headerTemplate =
         "Content-Disposition: form-data; name=\"{0}\"; filename=\"{1}\"\r\n" +
         "Content-Type: application/octet-stream\r\n\r\n";
     //Using the template write all files in your files[] input array to the request
     //"uplTheFile" is the destination file's name
     for (int i = 0; i < files.Length; i++)
     {
         memStream.Write(boundarybytes, 0, boundarybytes.Length);
         var header = string.Format(headerTemplate, "uplTheFile", files[i]);
         var headerbytes = System.Text.Encoding.UTF8.GetBytes(header);
 
         memStream.Write(headerbytes, 0, headerbytes.Length);
         //Convert files to byte arrays for upload
         using (var fileStream = new FileStream(files[i], FileMode.Open, FileAccess.Read))
         {
             var buffer = new byte[1024];
             var bytesRead = 0;
             while ((bytesRead = fileStream.Read(buffer, 0, buffer.Length)) != 0)
             {
                 memStream.Write(buffer, 0, bytesRead);
             }
         }
     }
 
     memStream.Write(endBoundaryBytes, 0, endBoundaryBytes.Length);
     request.ContentLength = memStream.Length;
     //Write the data through to the request
     using (Stream requestStream = request.GetRequestStream())
     {
         memStream.Position = 0;
         byte[] tempBuffer = new byte[memStream.Length];
         memStream.Read(tempBuffer, 0, tempBuffer.Length);
         memStream.Close();
         requestStream.Write(tempBuffer, 0, tempBuffer.Length);
     }
     //Capture the response from the server
     using (var response = request.GetResponse())
     {
         Stream stream2 = response.GetResponseStream();
         StreamReader reader2 = new StreamReader(stream2);
         return reader2.ReadToEnd();
     }
 }

Source StackOverflow

I ran into a similar problem. I think you need to change:

www.uploadHandler.contentType = "multipart/form-data";

to:

www.SetRequestHeader("Content-Type", "multipart/form-data");