def do_execute(self):
        """
        The actual execution of the actor.

        :return: None if successful, otherwise error message
        :rtype: str
        """
        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
The actual execution of the actor.

        :return: None if successful, otherwise error message
        :rtype: str