If by file format you mean an exported 3D model made in e.g. Blender, then use either FBX or OBJ. Unity can handle both, so it doesn't really matter which one you choose.
You can also make a map directly inside the Unity Editor without the need of a 3D modeling software by using ProBuilder (which is free) or UModeler (which costs $125).
Be sure to read this thread: https://warmerise.com/forums/topic/1906/custom-maps-2-0