1. Go to Policy & Objects > Policy > IPv4 (or IPv6, depending).

2. Edit or create a security policy.

3. Select a proxy-based security profile. You will know that there is a proxy component to the Security Profile because when a Security Profile is Proxy based the Proxy Options field will be visible (for example, select an Antivirus profile that includes proxy scanning).

4. Beside Proxy Options select the name of the MTU proxy options protocol.

5. Select OK to save the security policy.

6. Once you complete these steps, any files in the traffic subject to Security Profile scanning handled by this policy that are larger than 8MB will be blocked. If you have multiple firewall policies, examine each to determine if you want to apply similar file blocking the them as well.