// Cancel file transfer while it is in progress.
  public boolean cancelFile(String channelName, String exchangeId) {
    Log.d(TAG, TAGClass + "cancelFile()");
    // Request the channel interface for the specific channel name.
    IChordChannel channel = mChord.getJoinedChannel(channelName);
    if (null == channel) {
      Log.e(TAG, TAGClass + "cancelFile() : invalid channel instance");
      return false;
    }

    // @param exchangeId Exchanged ID
    return channel.cancelFile(exchangeId);
  }