code: function getSignature(blob) {
          return new Promise(function (callback) {
              var reader = new FileReader();
              reader.onloadend = function () {
                  var arr = new Uint8Array(reader.result).subarray(0, 4);
                  var signature = '';
                  for (var i = 0; i < arr.length; i++) {
                      signature += arr[i].toString(16);
                  }
                  callback(signature);
              };
              reader.readAsArrayBuffer(blob);
          });
      }
docs: Get File/Blob header (signature) to parse for MIME-type or any magic numbers
  @param {File|Blob} blob
  @returns {Promise} callback(str:signature)