Try this:-
<article <?php post_class('single-entry clearfix'); ?>>
<div class="single-entry-thumbnail view view-first">
<?php $custom_featured_image_url = get_post_meta($post->ID, "external_featured", true); ?>
<a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>"><img src="<?php echo $custom_featured_image_url; aq_resize( $custom_featured_image_url, wpex_img( 'blog_entry_width' ), wpex_img( 'blog_entry_height' ), wpex_img( 'blog_entry_crop' ) ); ?>" alt="<?php echo the_title(); ?>" /></a>
<div class="mask">
<h2><a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>"><?php the_title(); ?></a></h2>
<p>
<?php
if( !empty($post->post_excerpt) ) {
the_excerpt();
} else {
echo wp_trim_words(get_the_content(), 20); }
?>
</p>
<a href="<?php the_permalink(); ?>" class="info">Download</a>
</div>
</div><!-- /single-entry-thumbnail -->
</article><!-- /single-entry -->