3D mesh for paraview

I’ve saved some 3D data as a vtu file so that I can look at it in Paraview. However, in Paraview I am seeing only the solution along the boundary. When I try to use the volume filter, it states that I don’t have tetrahedra cells. From the Paraview forums, I’ve gathered that I’m not passing Paraview a 3D mesh.

Have I saved the data incorrectly? Do I have to do something special for 3D meshes? Below is my code.

load "iovtk"
load "msh3" 
load "medit" 

mesh3 Th = readmesh3("cube.mesh"); // saved mesh built with cube command

fespace Vh(Th, P1);
Vh u

u[] = readsol("u.sol"); // saved solution computed over above cube domain 

int[int] Order = [1];
string DN = "uData";
savevtk("u.vtu", Th, u, dataname=DN, order=Order);