아래처럼 하시면 확장자별로 이미지를 출력하실 수 있습니다.

<?php if(isset($content->attach->file1)):?>
<div class="kboard-attach">
    <?php $extension = strtolower(pathinfo($content->attach->file1[0], PATHINFO_EXTENSION));?>
    <img src="디렉토리경로/<?php echo $extension?>.png" alt="">
    <?php echo __('Attachment', 'kboard')?> : <a href="<?php echo $url->getDownloadURLWithAttach($content->uid, 'file1')?>"><?php echo $content->attach->file1[1]?></a>
</div>
<?php endif?>