Ever since Windows 3.1, the association between a file type (like png, or txt) and an application has been done through a few keys in the Windows Registry, as explained in the documentation on MSDN.

HKEY_CLASSES_ROOT
    .jpg
        (Default) = Bearly.jpg
    .Bearly.jpg
       (Default) = BearlyPhotoViewer
    BearlyPhotoViewer
       (Default) = "Bearly.exe" "%1"

But starting with Windows 8 and continuing in Windows 10, this is not enough anymore. Windows now allows the user to associate a file type with a certain application only via the built-in GUI. The above registry keys, although still necessary, are not enough anymore to achieve this association.

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