code: public MultipartBuilder addFormDataPart(String name, String filename, NetworkRequest value) {
          if (name == null) {
              throw new NullPointerException("name == null");
          }
          StringBuilder disposition = new StringBuilder("form-data; name=");
          appendQuotedString(disposition, name);
  
          if (filename != null) {
              disposition.append("; filename=");
              appendQuotedString(disposition, filename);
          }
  
          return addPart(new NetworkRequest.Builder()
                  .setBody(value.data)
                  .setHeaders(value.headers)
                  .addHeader("Content-Disposition", disposition.toString()).build());
      }
docs: Add a form data part to the body.