I just fixed this on my PC. Here is what I did:
Browse to the location of Notepad++. Copy the directory. Mine was
“C:\Program Files\Notepad++\notepad++.exe”
Run CMD as administrator.
Create a new file type to associate with file extensions. This command for me was…
ftype nppfile=“C:\Program Files\Notepad++\notepad++.exe” %1
Associate your file extension with the new file type. The command was…
assoc .nfo=nppfile
Now when I double-click on .nfo files it opens them in notepad++. Hope it works for you!