A JSON body is encoded as defined for the JSON message mapping defined in 220.127.116.11.4.7.
When sending a MQTT Version 5.0 message the MQTT ContentType property shall be set to application/json when sending uncompressed JSON messages.
JSON messages can become quite large. In order to save bandwidth and to reduce message size, on MQTT Version 5.0 the MQTT ContentType property allows to select a compression type as encoding for a JSON message.
When sending a gzip (RFC 1952) compressed JSON message on MQTT Version 5.0 the MQTT ContentType property shall be set to application/json+gzip.