If we're talking about right-click menus, Opus simply displays the menu items that the shell extension tells it to.
There are settings which can affect which shell extensions Opus asks, but if it's asking the TFS extension about folders then chances are it's also asking about files (unless you've gone really out of your way, but that's unlikely and you'd know if you had).
So for some reason the extension isn't returning any menu items for files. You would have to ask the extension's authors why that might be.
Checking what it does inside Notepad's File Open dialog is sometimes worthwhile, in case the extension only adds certain menu items when it's running inside certain processes.