app\models\Beatmapset::coverSizes PHP Method

    public static function coverSizes()
    {
        $shapes = ['cover', 'card', 'list'];
        $scales = ['', '@2x'];
        $sizes = [];
        foreach ($shapes as $shape) {
            foreach ($scales as $scale) {
                $sizes[] = "{$shape}{$scale}";
            }
        }
        return $sizes;
    }

 private static function isValidFormat($size)
 {
     return in_array($size, Beatmapset::coverSizes(), true);
 }