FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
Article Id 196992

This article describes how to show 'Packet Capture' under Log & Report -> Sniffer Traffic.

1) Create a new packet capture filter under: Network -> Packet Capture and select 'New'.
2) Put in respective filter on the interested traffic to capture and select 'Start'.
3) Login to FortiGate via SSH and execute the following command:
# config report setting
    append report-source sniffer-traffic
4) Run the traffic on a client and wait the packet capture to complete and the log is saved on the FortiGate under: Log & Report -> Sniffer Traffic.
Important Notes.
Local disk is required in order to work, as it rely on FortiView function.