Unable to view the changes in the thumbnail preview of Database browser.
Solution
When changes are made or a required option is selected for a preview type on the Database browser in defaults, you need to run Autoload to apply the settings.
Find below example screenshots of:
Create thumbnails in format: option set to 3D
Preview in the Design browser after Autoload taking effect
The preview will be available only if the formats selected are saved in the .ARD file.