Show all images attached to posts as a gallery page?

Question

I want a gallery page where all images will be shown, which were uploaded with all posts.

I tried this code with a page template, but the output of this code shows nothing, not even HTML code (excluding header and footer) in my browser:

<?php get_header();        
/*
Template Name:Gallery
*/        
while ( have_posts() ) : the_post();    
    if ( $post->post_type == 'data-design' && $post->post_status == 'publish' ) {
        $attachments = get_posts( array(
            'post_type'      => 'attachment',
            'posts_per_page' => -1,
            'post_parent'    => $post->ID,
            'exclude'        => get_post_thumbnail_id()
        ) );    
        if ( $attachments ) {
            foreach ( $attachments as $attachment ) {
                $class = "post-attachment mime-" . 
                    sanitize_title( $attachment->post_mime_type );
                $thumbimg = wp_get_attachment_link( $attachment->ID, 'thumbnail-size', true );
                echo '<li class="' . $class . ' data-design-thumbnail">' . 
                    $thumbimg . '</li>';
            }
        }
    }
endwhile;        
get_footer(); ?>

How can I make this page template?

Answer 1

Create page template with this code :

$args = array(
            'post_type' => 'attachment',
            'posts_per_page' => -1
        );
$query = new WP_Query( $args);
if($query->have_posts()){ //check if found any attachment
    while($query->have_posts()){ //loop
        $query->the_post();
        //you can add your code here 
    }
}else{
// if found NO posts
}