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 191455
This article describes how to debug managed FortiSwitch firmware upgrade 'Image file doesn't match platform' error on FortiGate as shown below:

Attempt FortiSwitch firmware upgrade using one of the following options:

1) Try upgrading FortiSwitch firmware using other web-browsers such as Microsoft Internet Explorer/Microsoft Edge/Chrome/Firefox and check the behavior if FortiSwitch firmware upgrade works via FortiGate GUI .

2) To speed up how fast the image is pushed from the FortiGate unit to the FortiSwitch units, on FortiGate CLI, enable the HTTPS image push instead of the CAPWAP image push and then try FortiSwitch upgrade:
# config switch-controller global
    show full
        set https-image-push enable
3) There’s a similar Bug 479692 – where FortiGate GUI prompts an error 'Image file doesn't match platform' when User is uploading correct FortiSwitch image on FortiGate GUI.This Bug is fixed in FortiOS v6.0.7 and v6.2.2 and later versions.

Consider upgrading FortiOS image after verifying compatibility:

4) If FortiGate firmware upgrade is not an option then FortiSwitch firmware upgrade can be done via FortiGate CLI using 'Firmware upgrade of stacked or tiered FortiSwitch units using stage upgrade procedure on FGT' - Refer Page 57: