The File Verification test examines your Form Tools Core (2.1.5 or later), modules, and themes to confirm that all the files exist on your server. If the test fails, it will list those files that are missing. You will need to download the appropriate component (and make sure you get the right version!) and manually upload them to the right spot on the server.
Please note that this test only checks for file existence. It doesn't actually check that the files contain the correct data. That can occasionally be the source of problems after upgrading, when the new files haven't been properly overwritten.