As it's Windows Server 2008, you can use Group Policy Preferences
Create/edit a GPO and add the file extension with the expected program by using
User Configuration\Preferences\Control Panel Settings\Folder Options
To make user configuration settings apply when linking GPO to OU containing computer objects, you nead to enable loopback processing in the following policy setting. If that isn't enabled, only computer configuration settings are applied when linking GPO to OU containing computer objects.
Computer Configuration\Policies\Administrative Templates\System\Group Policy\User Group Policy loopback processing mode