Default configuration

TYPO3 CMS comes with some default settings, which are defined in file typo3/sysext/core/Configuration/DefaultConfiguration.php.

Here is an extract of that file:

return [
        'GFX' => [ // Configuration of the image processing features in TYPO3. 'IM' and 'GD' are short for ImageMagick and GD library respectively.
                'thumbnails' => true,
                'thumbnails_png' => true,
                'gif_compress' => true,
                'imagefile_ext' => 'gif,jpg,jpeg,tif,tiff,bmp,pcx,tga,png,pdf,ai,svg',
                // ...
        // ...

You will probably find it interesting to take a look at that file, which also contains values not displayed in the Install Tool and thus not easily available for modification.