[Paraview] What vtk file formats may be volume rendered?

[Jeff Becker]

I have a binary data set on an irregular (non-evenly spaced) rectangular 
grid, that I read into a numpy array and then convert via pyevtk to vtk. 
Besides unstructured grid format, do any others allow the Volume 
(Render) option to be available in the representation pulldown in 
ParaView? I tried .vtr format and that didn't work. Thanks.

-jeff