code: public SftpMessage sendExtensionMessage(String request, byte[] requestData)
  			throws SshException, SftpStatusException {
  
  		try {
  			UnsignedInteger32 id = nextRequestId();
  			Packet packet = createPacket();
  			packet.write(SSH_FXP_EXTENDED);
  			packet.writeUINT32(id);
  			packet.writeString(request);
  
  			sendMessage(packet);
  
  			return getResponse(id);
  		} catch (IOException ex) {
  			throw new SshException(SshException.INTERNAL_ERROR, ex);
  		}
  	}
docs: Send an extension message and return the response. This is for advanced
  use only.
  
  @param request
  String
  @param requestData
  byte[]
  @return SftpMessage
  @throws SshException
  @throws SftpStatusException