Did you try closing the editor and reopening it or restarting your computer? As far as I know the editor should automagically import uasset files into the content browser.
I had the project on a later version of the engine, and the assets were opened and saved in the later version. I then downgraded to an earlier version. This is when the assets didn’t show up. They would fail to save.
I seem to be having a very similar problem, my player controller and everything i was working on last night refuses to open now… I had an earlier verison of the project opened last night to look at some things i did but it should not have interfered with anything. Or saved the player controller with the earlier version… But it sounds very much like what happened here, is there a way to fix it? the earlier version of my project i had open is very old: /