Custom Thumbnail for Material possible?

I am wondering if there are any ways of creating custom thumbnails for your Materials.
For example: Instead of using the normal Sphere, can I change this to a cube or a plane?

Here is what I know so far:

  • In the content browser I can modify the existing thumbnail by pressing the [Thumbnail edit mode].
  • In the Edit material window > Selecting the [Main Material Node - (If that makes any sense)], in the Details tab at the bottom there are a category named Thumbnail, it has the following subcategories:
    Primitive type
    Preview Mesh
    Orbit Pitch
    Orbit Yaw
    Orbit Zoom

Whenever I change the primitive type from TPT Sphere to something else, Apply and save the material, nothing happens.
If I then open up the material edit window again, I can see that the TPT Sphere is the selected primitive type again.
I have tried this on multiple materials already and nothing seems to work. Is this a bug or am I doing something wrong?
NOTE: I am currently working on 4.11.1 and this is a fresh new project created in 4.11.1.

Thanks for reading!