"Used with Particle Sprites" prevents material instances thumbnails from being editable

The thumbnails helpers will force material instances to be a plane, if their parent material is unedited.

This can be worked around by editing the parent material, but editing the material instance should override this behavior. 

1. Create a Material.
2. Create a Material Instance using the Material above as a parent.
3. Enable "Used with Particle Sprites" in the Usage of the parent Material.
4. The thumbnail of the Material Instance automatically cahnges to a plane.
5. Even if selecting another model than a plane in Thumbnail Edit Mode, nothing changes.