If you are cannot associate a file extension to an application in Windows 7/8/10, you may want to try this out:
Cannot associate a file extension
Open regedit and navigate to:
Here find the .exe name which is creating problems
Expand it and navigate it to:
Shell > Open > Command.
Double click on it and ensure that the application location points to the actual location of the executable file, in the value data field.
Exit the Registry Editor.
Now associate the file type for this particular extension and see if it helps!