getBlob(url, mimeType) {
    let encodedUrl = window.encodeURIComponent(url);
    return this.storage.getItem(encodedUrl).then(function (uint8array) {
      if (!uint8array) return;
      mimeType = mimeType || mime.lookup(url);
      return new Blob([uint8array], {
        type: mimeType
      });
    });
  }
Get a Blob from Storage by Url
@param  {string} url
@param  {string} [mimeType]
@return {Blob}