PowerRename is one of the tools which is part of Microsofts PowerToys tool collection. As it's name suggests you can rename files with it.

But what can you do, when you have a bunch of files without a proper file extension? It's also possible to add these to selected files. Just mark all relevant files and select PowerRename in the context menu after you made a right click with your mouse.

Make sure you've selected regular expression and then enter ($) in the search field. In the replace field just enter the file extension you want to add. For example .txt.

On the right side you should see a preview and when you're happy with it, just let the tool run and it will add the entered file extension to every file.