|if you type "assoc/?" at the command prompt it gives you help for the assoc command and it say:|
Microsoft Windows [Version 6.1.7600]
Copyright (c) 2009 Microsoft Corporation. All rights reserved.
Displays or modifies file extension associations
.ext Specifies the file extension to associate the file type with
fileType Specifies the file type to associate with the file extension
Type ASSOC without parameters to display the current file associations.
If ASSOC is invoked with just a file extension, it displays the current
file association for that file extension. Specify nothing for the file
type and the command will delete the association for the file extension.
In the last part of this it says "Specify nothing for the file
type and the command will delete the association for the file extension". I think this is your answer