为什么PCSMetaResponse.type 总是 Media_Unknown

Baidu-PCS-SDK-Android-L2-2.1.0：（百度个人云存储http://developer.baidu.com/ms/pcs）

为什么 BaiduPCSClient.meta()返回的 PCSMetaResponse.type 总是 Media_Unknown ？

    if(null != mbOauth){
        Thread workThread = new Thread(new Runnable(){
                    public void run() {
                        BaiduPCSClient api = new BaiduPCSClient();
                        api.setAccessToken(mbOauth);
                        String path = mbRootPath + "/ss.mp3";
                        final BaiduPCSActionInfo.PCSMetaResponse ret = api.meta(path);
                        Log.d("zy", "ret.type  =  "+ret.type);
                    }
                }
                workThread.start();
        }