/// <summary>
 /// Removes a stream from the file.
 /// </summary>
 /// <param name="streamId"></param>
 public override void RemoveStreamFromFile(string streamId)
 {
   var streamState = DocumentStreams.FirstOrDefault(s => s.Stream.id == streamId);
   if (streamState != null)
   {
     DocumentStreams.Remove(streamState);
     WriteStateToFile();
   }
 }