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If you cannot assign a file extension to an application under Windows 7/8/10, you should try this :
Unable to assign a file extension
and navigate to:
Here you will find the command. . exe name that causes problems
Expand and navigate to :
Shell > Open > Command.
double-click it and make sure that the application location points to the actual location of the executable file in the value data field.
Exit the registry editor.
Now link the file type for this special extension and see if it helps!
Our Freeware File Association Fixer for Windows should also interest some of you. This article shows you how to reset all file associations in Windows 10 to default settings.