FortiAP devices are thin wireless access points (AP) supporting the latest Wi-Fi technologies (multi-user MIMO 802.11ac Wave 1 and Wave 2, 4x4), as well as 802.11n, 802.11AX , and the demand for plug and play deployment.
Article Id 189901
This article describes how to go about - Unable to upgrade FortiAP via FortiGate GUI due to 'Image file doesn't match platform' error as shown below:

User Impact.  

FortiAP upgrades via FortiGate does not work.

Debug (possible cause and fix).

1) Ensure FortiAP firmware used is a valid image and belongs to right FortiAP model.
2) If FortiAP firmware image is correct, try FortiAP upgrade via FortiGate GUI using other web-browsers such as Microsoft Internet Explorer/Edge/Chrome/Firefox.
3) There’s a similar Bug 0479692 – where FortiGate GUI prompts an error 'Image file doesn't match platform' when User is uploading correct FortiAP image on FortiGate GUI – this Bug is fixed in FortiOS v6.0.8 and v6.2.2 and later versions.

Page 46.

4) If FortiGate upgrade is not an option then try upgrading FortiAP units from as shown in 'related articles'.

Related document.
FortiAP unit firmware upgrade.

Related Articles

Technical Note: Upgrading FortiAP Firmware