Answer by Christothes for How do you remove a default program association for file types in Windows 7?

Find the file extension in question under this key in the registry:

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts

Deleting the sub-key with the same name as the extension you want to un-associate will delete the default program association. In older Windows versions, you'll have to kill and restart explorer.exe for this to take effect (the change is instant as of Windows 10 64-bit).

In most cases, you will also need to remove the same sub key from HKEY_CLASSES_ROOT as well.