chooseVideoFiles: function chooseVideoFiles() {
      return new Promise(function (resolve, reject) {
        wx.chooseMedia({
          count: 9,
          mediaType: ['image', 'video'],
          sourceType: ['album', 'camera'],
          maxDuration: 50,
          camera: 'back',
          success: function success(res) {
            console.log(res);
            var videoFiles = res.tempFiles.map(function (file) {
              return file.tempFilePath;
            });
            console.log(videoFiles);
            resolve(videoFiles);
          },
          fail: function fail(err) 
选择多个视频，只能成功一个

{"tempFiles": [{"height": 1280, "thumbTempFilePath": "wxfile://tmp_9bjpg", "width": 592, "size": 2254124, "duration": 17, "tempFilePath": "wxfile://tmp_6cbxxx.mp4", "fileType": "video"}], "errMsg": "chooseMedia:ok", "type": "video", "failedCount": 1}