As mentioned in https://itch.io/t/3005131/how-i-see-the-thumbnail-on-the-file, native support for video thumbnails is a planned feature. However, it's been over 2 years with essentially no update on this. Is there any timeline for this feature?

Maybe during/after the big UI overhaul and update to UE5. Still finishing Apple Vision Pro port right now (been working on that for the past year). My time estimates are pretty awful and meaningless at this point. There's a ton of work that needs to be done for the UE5 update.