code: func createFormFile(filename string, w *multipart.Writer, contentType string) (io.Writer, error) {
  	h := make(textproto.MIMEHeader)
  	h.Set("Content-Disposition",
  		fmt.Sprintf(`form-data; name="filedata"; filename="%s"`,
  			escapeQuotes(filename)))
  	h.Set("Content-Type", contentType)
  	return w.CreatePart(h)
  }
docs: // copied from multipart package with slight changes due to fixed content-type there