<?php foreach($content->attach as $key => $attach):?>
                    <p class="thumbnail-area"><img src="<?php echo get_site_url() . $attach[0]?>" alt="<?php echo $attach[1]?>"></p>
<?php endforeach?>

그 위에

<?php if($content->thumbnail_file):?>
                    <p class="thumbnail-area"><img src="<?php echo get_site_url() . $content->thumbnail_file?>" alt=""></p>
<?php endif?>

이 코드를 추가 해주세요 :)