there are difference between explorer and DO in thumbnail view.
I work in an automobile engineering and design department. i use software called CATIA and create 3D file with *.catpart extensions.
in explorer, i can see preview of *.catpart file but nothing in DO. 
Is the "use shell image extraction" option turned on in your thumbnail Preferences? That should, in theory, mean that any thumbnails in Explorer will also appear in Opus.
Oh yes !!! really sorry !!!
i use the french language pack and this option is not really correctly translated. 