Hi James, yes thumbnails in finder windows is supported in MoI v5.

Check that it hasn't been turned off in MOI under Options > Import/Export > 3DM options > "Write thumbnail preview image" checkbox.

Also you may need to enable the extension in macOS System Settings, under under General > "Login Items & Extensions". There should be an entry there for the v5 app and when you click the circle with an "i" in it there should be a switch that you can turn on.

- Michael