An increasing number of computer viruses have been designed to spread via file attachments. To protect your email system, configure a list of file types that are unauthorized for all accounts.
Messages with those type of files attached are rejected. By default the recipient and the sender are notified that the message was blocked. If you do not want to send a notification to the recipient when messages are blocked, you can disable this option from the Global Settings > Attachments page.
Procedure
Go to the Configure > Global Settings > Attachments page.
In the Common Extensions list, select the extensions to block.
Click Add Selected or Add All.
If an extension that should be blocked is not listed, in the New extension text box, type the extension and click Add.
Click Save.