An increasing number of computer viruses have been designed to spread via file attachments. To protect your email system, you can configure a list of file types that are unauthorized for all accounts. This is done from the Global Settings, Attachments tab.
The most common file types are listed and you can add other extension types to the list.
Messages with those type of files attached are rejected and the sender gets a bounce notice. The recipient does not get the mail message and is not notified.