For some reason my files have all turned into .uasset? Unreal doesn't even recognise the format. I don't know what's happened. Help?

I’ve removed my files manually in windows explorer, out of Unreal, to put them into another project. For some reason, it wouldn’t accept them because it doesn’t understand the format .uasset. Sure enough, when I looked, my files were .uasset. No clue what this format is, how they are this format, how long they have been or why. Anyone know?

EDIT: I know what these are. I still don’t understand why Unreal won’t accept them. I’ve tried dragging them in, importing them, everything. It doesn’t recognise the format.

I have worked it out. To any who want to know, you can’t copy in or drag in .uasset files. You can only have them in another project, and ‘migrate’ the folder they are in, to your other projects ‘contents’ folder. Hope this helped others!