It’s not quite self hosted, but Soulseek allows you to share share private directories with buddies. Soulseek might require a port forward.

Other than that, there are the many pasteboard solutions that have been mentioned. They’ll either require a port forward or reverse proxy (nginx etc.) to access outside the network though.

For me, writing like this is always read with the AI voice from Satisfactory. And it slowly gets more and more corrupted.