Using NPP6.6.7 Win7 64-bit, when I go in to settings/Preferences and select File Association, I add file extensions (e.g., select Notepad, that shows .txt, .log, .ini). I add to the registered extensions (by selecting and clicking '->'). I then click close.
When I close NPP and go back in, those extensions are gone.
To my memory you must start Notepad++ as Administrator to change this setting permanently.
If you do not like this you can assign the extension via the windows explorer (see attachments taken on a German Windows 8.1):
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