Change line 31 in application/views/dashboard/widget_documents.php to:
</a>
<?php if ($document->isDisplayable()) {?>
<div>
<div style="position: relative; left:0; top: 0; width: 450px; height: 120px; background-color: white">
<iframe style="width:100%;height:100%;border:1px solid #ddd;" src="<?php echo get_sandbox_url("feed", "display_content", array("id" => $document->getId(), "user_id" => logged_user()->getId(), "token" => logged_user()->getTwistedToken())) ?>"></iframe>
</div>
</div>
<?php } elseif (($dtype = $document->getFileType()) instanceof FileType && $dtype->getIsImage()) { ?>
<div>
<a href="<?php echo get_url('files', 'download_image', array('id' => $document->getId(), 'inline' => true)); ?>" target="_blank" title="<?php echo lang('show image in new page') ?>">
<img id="<?php echo $document->getId(); ?>Image" src="<?php echo get_url('files', 'download_image', array('id' => $document->getId(), 'inline' => true)); ?>" style="max-width:450px;max-height:120px"/>
</a>
</div>
<?php } // if ?>
</td>
This enables previews for documents as well as images. As I have not that much experience with the object's inner behavior I simply copied some stuff from the files detail template and adapted it accordingly. If there are any improvements for this code please let me know.