MQTTnet Broker Payload Encoding

Question

I use MQTTnet broker:

    _mqttServer = mqttFactory.CreateMqttServer(mqttServerOptions);
    _mqttServer.ClientConnectedAsync += MqttServer_ClientConnectedAsync;       
    _mqttServer.InterceptingPublishAsync += args =>        
    {  
        _logger.LogInformation("Message received: {0}", Encoding.UTF8.GetString(args.ApplicationMessage.PayloadSegment));
        return CompletedTask.Instance;            
    }; await _mqttServer.StartAsync(mqttServerOptions);       

The connection works fine and I'm able to get a message from a client with a payload but I'm not able to encode the message. Here is an example payload:

    { 209, 4, 37, 0, 0, 0, 65, 56, 48, 48, 48, 95, 67, 80, 56, 48, 53, 120, 95, 116, 101, 115, 116, 95, 99, 108, 105, 101, 110, 116, 95, 71, 70, 50, 50, 48, 55, 48, 55, 49, 51, 54, 54, 1, 232, 3, 235, 1, 222, 3, 217, 203, 211, 44, 217, 203, 211, 44, 1, 0, 1, 0, 0, 0, 8, 0, 192, 175, 112, 32, 148, 12, 218, 1, 0, 0, 176, 65 }

using UTF-8 I get:

�%???A8000_CP805x_test_client_GF2207071366������,���,?????��p ��??�A

I have no control what is sent by a client but I have been told that it is a json message. I haven't found any solution to encode it into a json format.

Please help.