Hi Finema, I updated to Tahoe 26.1 to test it and the thumbnails are working ok for me over here.

Could you post an example .3dm file that is not getting the thumbnail so I can verify that it has the thumbnail image in it?

Also the system setting UI for enabling the extensions is kind of confusing.

Under System Settings, it's under General > "Login Items & Extensions" and then to enable it you need to click the circle with an "i" in it which doesn't really look like a clickable thing. Then it will show the enable/disable switch.

So please double check that it is enabled.

- Michael