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Summary: How to make sure the Rhino thumbnail viewer is registered in Windows
If you do not see preview thumbnails of Rhino .3dm files in Windows Explorer, the follow these steps.
If you have Rhino 6 for Windows installed, then run the following command from the Command Prompt window you just opened:
regsvr32 "C:\Program Files\Rhino 6\System\RhinoHandlers.dll"
If you have Rhino 5 for Windows (and you don't have Rhino 6 for Windows installed), then run the following command from the Command Prompt window you just opened:
regsvr32 "C:\Program Files\Rhinoceros 5 (64-bit)\System\RhinoHandlers.dll"