assets/mjcf/cube.xml

<?xml version="1.0" ?>
<mujoco model="cube">
  <asset>
    <mesh name="mesh_cube" file="../obj/cube.obj"/>
  </asset>
  <worldbody>
    <body name="cube">
      <geom name="cube_geom" type="mesh" mesh="mesh_cube"/>
    </body>
  </worldbody>
</mujoco>