How to use open3d.io.write_octree (filename, octree) to save an octree map?

[WrndCCX]

Checklist

• I have searched for similar issues.
• For Python issues, I have tested with the latest development wheel.
• I have checked the release documentation and the latest documentation (for main branch).

My Question

How to use open3d.io.writable octree (filename, octree) to save an octree map?
Do anyone have a solution?

My code is:
octree = open3d.geometry.Octree(max_depth=8)
octree.convert_from_point_cloud(voxel_pcd, size_expand=0.01)
open3d.visualization.draw_geometries([octree],window_name="Octree")
open3d.io.write_octree("octomap.ot", octree)
#open3d.io.write_octree("octomap.bt", octree)

The program is able to draw the window of an octree normally, but an error message appears when saving:
[Open3D WARNING] Write geometry::Octree failed: unknown file extension.