I've searched but can't see anything directly relevant. Can I set Dopus to show any files that I tell it are image files as thumbnails by default.

So, for example, if I open a folder of .rw2, .dng. .jpg images, it will show them as thumbnails. But not gif or png.

Yes, it's actually quite easy.

Add a new File Type Group (from the File Type window) with the extensions you want it to pick up (.rw2, .dng, .jpg etc). Name this group whatever you want.

Now go under Settings -> Folders -> Folder Format -> Content Type Formats. You'll see your newly created Group here. Make sure it's check-marked ON. Select and Edit to change it's format to be Thumbnails (or any other format you want).

Also make sure Settings -> Folders -> Folder Behaviour -> "Enable Folder Content Type detection for..." is enabled and set correctly.

That should be it. Hope this helps.

Well it doesn't so much help as answer my query in full - thanks!

brilliant ... helped me with my query ...