[Solved] Mqtt with aqara contact sensor (zigbee2mqtt)

I think you would need to chain JSON and transformation map transforms for this.
Here is a post talking about it with an example (JSONPATH:$.STA∩MAP:statizone.map).
Never tried it myself though.

Edit: in your case the chain would be JSONPATH:$.contact∩MAP:contact.map (or however you define the map file).