It’s messy, but if you setup both a Post and a Topic webhook. When a message, or topic is first created, both hooks will be fired. You can use the topic_id that is sent with the Post webhook to get the topic archetype from the topic webhook data. It will be set to ‘private_message’ for messages, ‘regular’ for regular topics.
It would be great to get some topic data sent with the Post webhook - category_id and archetype would both be useful.