SOLVED!

My fault, found the mistake. Browser translate the code in german ... 🤣 

---

add_filter('wpforo_content_after', 'wpforo_default_attachment_image_embed', 11);
function wpforo_default_attachment_image_embed( $content ){
    if( preg_match_all('|<a class=\"wpforo\-default\-attachment\" href\=\"([^\"\']+)\"[^><]*>.+?<\/a>|is', $content, $data, PREG_SET_ORDER) ){
        foreach($data as $array){
            if(isset($array[1])){
                $file = $array[1];
                $e = strtolower(substr(strrchr($file, '.'), 1));
                if( $e === 'jpg' || $e === 'jpeg' || $e === 'png' || $e === 'gif' || $e === 'bmp' || $e === 'webp' ){
                    $filename = explode('/', $file); $filename = end($filename);
                    $html = '<a href="' . esc_url($file) . '" target="_blank"><img class="wpforo-default-image-attachment" src="' . esc_url($file) . '" alt="' . esc_attr($filename) . '" title="' . esc_attr($filename) . '" /></a>';
                    $content = str_replace($array[0], $html, $content);
                }
            }
        }
    }
    return $content;
}