def extract(self):
        """Start extracting the files in the file list one by one using a
        new thread. Every time a new file is extracted a notify() will be
        signalled on the Condition that was returned by setup().
        """
        self._extract_thread = threading.Thread(target=self._thread_extract)
        self._extract_thread.setDaemon(False)
        self._extract_thread.start()