Interesting...
Try this:
1. Start a registry editor (e.g., regedit.exe).
2. Navigate to the HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\policies\Explorer
registry subkey to configure the computer for all users or navigate to the HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies
registry subkey to configure the computer for the current user. If either subkey
doesn't exist, open the Edit menu and select New, Key to create it.
3. From the Edit menu, select New, DWORD Value.
4. Enter the name NoFileAssociate.
5. Set the value to 1 to disable the user's ability to change file associations (this
setting doesn't affect Power Users and Administrators); a value of 0 or a missing
value enables the user's ability to change file associations.
6. Click OK.
7. Close the registry editor.
8. Restart the computer for the changes to take effect.