To block attachment with certain extension make following steps:

1. In the IMSVA web console go to the Policy > Policy List page.
2. Click Add and select Other on the menu.
3. Define the appropriate settings on Select Recipients and Senders screen and click Next.
4. On the Select Scanning Conditions screen select Name or extension checkbox and input the necessary settings.
5. Prepare a plain text file that includes your extension.  *.wsf as an example.
6. Select Attachment Named checkbox and import prepared text file using the Import button.
7. The setting should look like this:

   

8. Click Save.