public void MarkAsIndex(string fileId)
        {
            var finalPath = FilePathSanitizer.CreateCleanlyJoinedFilePath(_dropFolder,
                                                                          fileId + ".idx");

            if (!File.Exists(finalPath))
            {
                throw new FileNotFoundException(fileId);
            }


            File.SetAttributes(finalPath, FileAttributes.ReadOnly);
        }