code: public File asFile(String fileName) throws HelloSignException {
          Integer lastResponseCode = getLastResponseCode();
          File f = null;
          try {
              if (lastResponseCode != null && lastResponseCode != HttpURLConnection.HTTP_OK) {
                  this.asJson(); // Will validate response
              }
              f = createTemporaryFile(fileName);
              if (getLastResponseAsFile(f) == 0) {
                  logger.warn("No bytes written to file: " + fileName);
              }
          } catch (HelloSignException ex) {
              throw ex;
          } finally {
              reset();
          }
          return f;
      }
docs: Executes the request and returns the response as a File.
  
  @param fileName String name of destination file
  @return File response
  @throws HelloSignException thrown if there is a problem executing the
  request