读取文件

 return new Promise((resolve, reject) => {
        let reader = new FileReader();
        reader.readAsArrayBuffer(file);
        reader.onload = function (evt) {
          let rawData = evt.target.result;
          let dataView = new DataView(rawData);
          var num = dataView.getUint32(0) & 0xffff; //读取文件的表盘编号
          resolve(num);
        };
      });