Troubleshooting Tip: The upload file is too big or invalid error with FortiGate Replacement Image

Scope

FortiGate has the option to use custom replacement images within replacement messages. Replacement images are useful for customizing certain login pages and block pages that have been generated from the FortiGate. In some cases, it is possible to run into an error when uploading a custom replacement image file.

1. Go under FortiGate GUI -> System -> Replacement Messages and select 'Manage Images'.

2. Select Upload Image and attempt to upload the desired image.

3. It is possible to run into the following error below.

Error:

"Upload file is too big or invalid."

When this error occurs, it's possible to utilize some CLI commands to allow for the image to get uploaded.

Note:
FortiOS only supports a maximum image file size of 24kb.

config system replacemsg-image

    edit <image_name>
        set image type {gif | jpg | tiff| png}
        set image-base64 <string>
    next
end