Open the file explorer, and in the Extras menu you have "folder options", the tab "File types" gives you a list with all known file extensions and their respective description. You are allowed to associate one program with a file type for opening.

The settings are stored in the windows registry under the key HKEY_CLASSES_ROOT\.extension. The "Standard" key's value is most times "extensionfile" and means that the settings of the key HKEY_CLASSES_ROOT\extensionfile shall be used. That allows to associate multiple file extensions with the same settings, but is a step too much if you have just one extension.