PnP_Nintex_Form_Controller.js

NWF$(document).ready(function () {

    $pnp.setup({
        sp: {
            headers: {
                "Accept": "application/json; odata=verbose"
            }
        }
    });

    var isDisplayMode = document.location.pathname.indexOf("/DispForm.aspx") > -1;

    if (isDisplayMode) {
        showAttachments();
    }
});


function showAttachments() {

    /* get item ID from URL */
    var urlParams = new URLSearchParams(window.location.search);
    var itemId = urlParams.get('ID');

    var item = $pnp.sp.web.lists.getByTitle("SimpleList").items.getById(itemId);
    var attachmentsTarget = NWF$('#tbxAttachments');

    item.attachmentFiles.select("FileName,ServerRelativeUrl").get().then(function (itemAttachments) {

        var attachments = itemAttachments.map(function (result) {
            return NWF$('<img/>', {
                src: result.ServerRelativeUrl,
                alt: result.FileName,
                style: "margin:5px;width:100px;height:100px;float:left;",
                class: "item-attachment"
            });
        });

        /* add the images to the target element */
        attachments.forEach(function (element) {
            element.appendTo(attachmentsTarget);
        });

        /* add click event for each image */
        NWF$(".item-attachment").each(function (index) {
            NWF$(this).click(function () {

                Swal.fire({
                    imageUrl: NWF$(this).attr('src'),
                    imageAlt: NWF$(this).attr('alt')
                })
            });
        });
    });
}