WordPress: Show post's attachment images

functions.php

<?php
// Show post's attachment images
function show_attachment_images() {
	$attachments = get_posts( array(
		'post_type' => 'attachment',
		'posts_per_page' => -1,
		'post_parent' => 42,
		'exclude'     => get_post_thumbnail_id()
	) );
 
	if ( $attachments ) {		
		echo '<div id="fullscreen-bg" class="group">'; 
			foreach ( $attachments as $attachment ) {
				$class = "post-attachment mime-" . sanitize_title( $attachment->post_mime_type ); 
				$img = wp_get_attachment_image( $attachment->ID, 'full' ); 
				echo $img;
			} 
		echo '</div>'; 
	}
}