/// <summary>
 /// Retrieve download link of the file on the cloud.
 /// </summary>
 /// <param name="fileStorage">A FileStorage object contains file's information on the cloud</param>
 /// <returns>Download link of the document.</returns>
 public Task <string> GetDownloadLink(FileStorage fileStorage) => storageManager.DownloadLink(fileStorage.FileId, "fileid");