code: function (readStream, callback) {
      var md5 = crypto.createHash('md5');
      readStream.on('data', function (chunk) {
          md5.update(chunk);
      });
      readStream.on('error', function (err) {
          callback(err);
      });
      readStream.on('end', function () {
          var hash = md5.digest('hex');
          callback(null, hash);
      });
  }
docs: 获取文件 md5 值