if (backupFolder != null)
{
    List<string> fileTypeFilter = new List<string>
    {
        ".txt",
        ".rtf"
    };

    QueryOptions queryOptions = new QueryOptions (CommonFileQuery.OrderBySearchRank, fileTypeFilter);
    StorageFileQueryResult queryResult = backupFolder.CreateFileQueryWithOptions(queryOptions);
    IReadOnlyList<StorageFile> files = await queryResult.GetFilesAsync();

    foreach (StorageFile file in files)
    {

    }
}