Troubleshooting Tip: FTPS file transfer is not working

This article describes a situation with FortiGate units running FortiOS 7.2.10 and later when operating in Flow mode with IPS or Deep Inspection enabled, and the FTPS file transfer is not working. 

When using explicit FTPS, after the TLS AUTH negotiation, the data channel (high-port session) is initiated.
Because the traffic is encrypted, the kernel session helper cannot extract the required session information and fails to create the expected session. As a result, the FortiGate denies the data connection.