Steps to reproduce:

1. Open Unity;
2. Create new project;
3. Import attached 'testBroadleafPreview.unitypackage';
4. Select 'Broadleaf_Desktop' gameobject in the Hierarchy view;
5. Click on any LODGroup level in Inspector (e.g. 'LOD0');
6. Observe LOD renderer preview.

Actual result:
LOD renderer preview is not displayed for .spm model - only grey square is rendered (see attached actual.png screenshot).

Expected result:
LOD renderer preview should be displayed .spm model . Expected behaviuor could be found while adding any gameobject to LODGroup (e.g. Cube) (see attached expected.png). LOD preview icon could be found in Project window - when expanding .spm prefab (see attached LODPreviewIcon.png screenshot).

Notes:
- Reproducible in: 5.4.0b5, 5.4.0b4, 5.3.2f1, 5.3.1f1;
- Reproducible on both OSX 10.11.3 and Windows 10.