TYPO3 CMS comes with some default settings, which are defined in
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.