code: public File getFiles(String requestId, String format) throws HelloSignException {
          if (format == null || format.isEmpty()) {
              format = FILES_FILE_EXT;
          }
          String url = BASE_URI + SIGNATURE_REQUEST_FILES_URI + "/" + requestId;
          String fileName = FILES_FILE_NAME + "." + format;
          return httpClient.withAuth(auth).withGetParam(PARAM_FILE_TYPE_URI, format).get(url).asFile(fileName);
      }
docs: Retrieves the file associated with a signature request.
  
  @param requestId String signature request ID
  @param format String format, see SignatureRequest for available types
  @return File
  @throws HelloSignException thrown if there's a problem processing the
  HTTP request or the JSON response.