No, the issue is that an application needs to implement the preview widget and add it to the GTK file picker, hence this work needs to be duplicated by every ap

No, the issue is that an application needs to implement the preview widget and add it to the GTK file picker, hence this work needs to be duplicated by every application and of course most applications don't do that. But if the application adds a preview widget it should work regardless of which view mode is active. Edit: Also the new GTK API for file pickers to work better with sandboxing and platform integration, lacks the ability to add preview widgets altogether.