code: def do_execute(self):
          
          fname = str(self.input.payload)
          if not os.path.exists(fname):
              return "File '" + fname + "' does not exist!"
          if not os.path.isfile(fname):
              return "Location '" + fname + "' is not a file!"
          if self.resolve_option("use_custom_loader"):
              self._loader = self.resolve_option("custom_loader")
          else:
              self._loader = converters.loader_for_file(fname)
          dataset = self._loader.load_file(fname, incremental=bool(self.resolve_option("incremental")))
          if not self.resolve_option("incremental"):
              self._output.append(Token(dataset))
          else:
              self._iterator = self._loader.__iter__()
          return None
docs: The actual execution of the actor.
  
          :return: None if successful, otherwise error message
          :rtype: str