Adds thumbnail of attachment to admin columns and image preview to CPT admin post page. CPT in example is 'guest-photos'. Using this in addition to "Frontend Uploader" plugin (https://wordpress.org/plugins/frontend-uploader/)

attachment-cpt.php

/* ===========================================
    Show Guest Photo Attachment in CPT Admin Columns
   =============================================*/

    // GET ATTACHED IMAGE
    function revcon_get_attachment_image($post_ID) {

        $images = get_attached_media('image', $post_ID); 

        if ($images) {
            foreach ($images as $image) { 
                $attachment_id = $image->ID;
                $thumbnail_url = wp_get_attachment_image( $attachment_id, array(60,60) );
                return $thumbnail_url;
            }
        }
    }
    

    // ADD NEW COLUMN TO ADMIN
    function revcon_columns_head($defaults) {
        $defaults['attachment_image'] = 'Photo';
        return $defaults;
    }

     
    // SHOW THE ATTACHMENT IMAGE IN COLUMN
    function revcon_columns_content($column_name, $post_ID) {
        if ($column_name == 'attachment_image') {
            $post_featured_image = revcon_get_attachment_image($post_ID);
            if ($post_featured_image) {
                echo $post_featured_image;
            }
        }
    }


    add_filter('manage_guest-photos_posts_columns', 'revcon_columns_head'); // "manage_guest-photos_posts_columns" will show this ONLY on the CPT guest-photos
    add_action('manage_guest-photos_posts_custom_column', 'revcon_columns_content', 10, 2);



/* ===========================================
    Show Guest Photo Attachment on CPT Post in Admin
   =============================================*/

    // GET ATTACHED IMAGE
    function revcon_render_attachment_metabox( $post ) {
        $output = '';

        $images = get_attached_media('image', $post->ID); 

        if ($images) {
            foreach ($images as $image) { 
                $attachment_id = $image->ID;
                $thumbnail_url = wp_get_attachment_image( $attachment_id, 'medium' );
                $output = $thumbnail_url;
            }
        }

      echo $output;
    }


    // ADD METABOX TO CPT
    function revcon_add_attachment_thumbs_metabox (){

        add_meta_box ('att_thumb_display', 'Attachmed Images','revcon_render_attachment_metabox','guest-photos', 'normal', 'high');

    }


    add_action( 'add_meta_boxes', 'revcon_add_attachment_thumbs_metabox' );