code: public String updateTemplateFiles(String existingTemplateId, TemplateDraft newTemplate, String clientId)
              throws HelloSignException {
          String url = BASE_URI + TEMPLATE_UPDATE_FILES_URI + "/" + existingTemplateId;
          HttpClient client = httpClient.withAuth(auth).withPostFields(newTemplate.getPostFields());
          if (clientId != null) {
              client = client.withPostField(EmbeddedRequest.EMBEDDED_CLIENT_ID, clientId);
          }
          Template t = new Template(client.post(url).asJson());
          return t.getId();
      }
docs: Replaces the backing documents for the given template with the given
  files.
  
  Note that certain rules apply to this endpoint:
  https://app.hellosign.com/api/reference#update_template_files
  
  @param existingTemplateId String ID of the template from which the
  overlay data (signatures, text fields, etc.) will be retrieved.
  @param newTemplate TemplateDraft that holds the data and documents which
  will be used as the basis for the new template. The following
  fields can be set in this request: files or file_urls, subject,
  message, and test_mode.
  @param clientId String optional ID of the app which is generating this
  new template. Set to null if not used.
  @return String ID of the template to be created
  @throws HelloSignException thrown if there is a problem processing the
  HTTP request