code: function _encodeBlob(blob) {
      return new Promise$1(function (resolve, reject) {
          var reader = new FileReader();
          reader.onerror = reject;
          reader.onloadend = function (e) {
              var base64 = btoa(e.target.result || '');
              resolve({
                  __local_forage_encoded_blob: true,
                  data: base64,
                  type: blob.type
              });
          };
          reader.readAsBinaryString(blob);
      });
  }
docs: encode a blob for indexeddb engines that don't support blobs