This line is your data in Base64 format - when you decode it you will have the byte array that your device sent. TTS can’t automagically do this because what you send is coded by you but you can tell it what the format is & decode it with a payload formatter.
Check out the documentation (link bottom right of every console page), particularly https://www.thethingsindustries.com/docs/reference/data-formats/#uplink-messages and https://www.thethingsindustries.com/docs/integrations/payload-formatters/ plus this older info which is still relevant https://www.thethingsnetwork.org/docs/devices/bytes/
I’ve moved your post from a topic unrelated to MQTT & JSON to a new one - and formatted your post using the </> tool on the toolbar - please see formatting guidelines