How to save voxel data

Hi All,

I created voxelgrid from Lidar point cloud data by (open3d.geometry.VoxelGrid.create_from_point_cloud).

My task is to save this voxel data in file by (open3d.io.write_voxel_grid()) and open it by some Viewers(ex. CloudCompare).

pointcloud = open3d.io.read_point_cloud(‘pointcloud.ply’)
voxel = open3d.geometry.VoxelGrid.create_from_point_cloud(pointcloud, voxel_size=5)
open3d.io.write_voxel_grid(‘voxelfile.***’, voxel)

What file extension I can use?(I woud like to know what is *** ↑)
I searched for it and nothing came up.
(I found out that I can use .ply . However, it is saved as pointcloud or mesh, not Voxel. )

I really appreciate if anyone can help. Thanks.