upload.php

<?php

add_filter('attachment_fields_to_edit', function ($fields, $post) {
    if (realpath($_SERVER['SCRIPT_FILENAME']) === ABSPATH.'wp-admin/async-upload.php') {
        $upload_default = get_option('dxw_members_only_upload_default');

        $fields['dmo_add_to_list'] = [
            'input' => 'html',
            'label' => __('Add to whitelist', 'dxwmembersonly'),
            'html' => '<input type="checkbox" name="attachments['.$post->ID.'][dmo_add_to_list]" '.($upload_default==='true'?'checked':'').'>',
        ];
    }

    return $fields;
}, 10, 2);

add_filter('attachment_fields_to_save', function ($post, $attachment) {
    if (isset($attachment['dmo_add_to_list']) && $attachment['dmo_add_to_list']) {
        $url = parse_url($attachment['url'], PHP_URL_PATH);

        $list_content = get_option('dxw_members_only_list_content');
        $list_content .= "\n" . $url;
        update_option('dxw_members_only_list_content', $list_content);
    }

    return $post;
}, 10, 2);