code: @Override
      protected HttpURLConnection getConnection() throws HelloSignException {
          if (fields != null) {
              for (String key : fields.keySet()) {
                  if (fields.get(key) instanceof File) {
                      return postWithFile();
                  }
              }
          }
          return postQuery();
      }
docs: Helper method to make an HTTP POST request. Intelligently detects whether
  Files have been attached and sends as an multipart form request.
  
  @return HttpUrlConnection
  @throws HelloSignException thrown if there is a problem making the HTTP
  request or processing the result