public function is_valid_image() {
        $mimetype = $this->get_mimetype();
        if (!file_mimetype_in_typegroup($mimetype, 'web_image')) {
            return false;
        }
        if (!$info = $this->get_imageinfo()) {
            return false;
        }
        if ($info['mimetype'] !== $mimetype) {
            return false;
        }
        // ok, GD likes this image
        return true;
    }