docs: def parse(self, fail_callback=None):
          """ Parse text fields and file fields for values and files """
          # get text fields
          for field in self.field_arguments:
              self.values[field['name']] = self.__get_value(field['name'])
              if self.values[field['name']] is None and field['required']:
                  if fail_callback is not None:
                      fail_callback()
                  self.__invalid_request(field['error'])
          # get file fields
          for file in self.file_arguments:
              self.files[file['name']] = self.__get_file(file)
              if self.files[file['name']] is None and file['required']:
                  if fail_callback is not None:
                      fail_callback()
                  self.__invalid_request(file['error'])