When you install NoteTab, the setup program makes only the minimum required changes to the registry (which many users appreciate). As a result, it does not configure your Windows system for file associations with NoteTab.
But the solution is really simple because file associations are managed directly from within NoteTab. Simply open the Options dialog box (you’ll find the command at the bottom of the View menu), click on the Associations tab, and then add extensions (without the dot) one by one.
Alternatively, you can set associations in Windows. See below for instructions for Windows 7 and Windows 10.