How Gmsh can creat hexahedra by save as .mesh file

Hi

I'm using SfePy to solve a problem.Firstly, I creat a simple cube by Gmsh and save as my.mesh file. Now I am using sfepy/examples/multi_physics/biot.py to calculate.but it something wrong. Then I open the "/meshes/3d/cube_medium_hexa.mesh",in the "cube_medium_hexa.mesh" file,it consists Vertices and Hexahedra.But in my mesh file,it only consists Vertices,Edges and Triangles,no Hexahedra. When I run the biot.py, there aren't any results I want. How can I save .mesh file including Hexahedra? I think it is the main probelm.

Regards, Kid