Description
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:

Solution
FortiAP upgrades via FortiGate does not work.
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.
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:

Solution
User Impact.
Debug (possible cause and fix).
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.
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