TYPO3 Core

Steps to reproduce:

1. Adjust $GLOBALS['TYPO3_CONF_VARS']['GFX']['imagefile_ext'] to exclude some extensions, e.g. set it to $GLOBALS['TYPO3_CONF_VARS']['GFX']['imagefile_ext'] = 'gif';
2. Open the "Image Processing" of the "Install Tool > Environment"
3. Run the checks
4. None allowed file extensions are stuck on "Loading …" instead of showing the result

Expected behaviour:

The sections of none allowed extensions should show the AJAX response

{
    "status": [
        {
            "severity": 1,
            "title": "Skipped test",
            "message": "Handling format png must be enabled in TYPO3_CONF_VARS['GFX']['imagefile_ext']",
            "storeInSession": false
        }
    ]
}

I could reproduce the issue in v11 and v12.