There is no direct support in Notion for Freeplane mind maps.

I came up with (slightly clunky) workaround so you can embed the map mm file alongside the exported image.

It should work for any third party tool, not just Freeplane

I documented it in Notion of course

https://alecthegeek.notion.site/Embed-unsupported-content-in-Notion-e78c086e49de4620a5951a398dbea5cc