magento-test/app/code/Magento/Theme/view/adminhtml/templates/tabs/js.phtml

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */

/**
 * @var $block \Magento\Theme\Block\Adminhtml\System\Design\Theme\Edit\Tab\Js
 * @var \Magento\Framework\View\Helper\SecureHtmlRenderer $secureRenderer
 */
?>
<?= $block->getFormHtml() ?>

<?php $scriptString = <<<script

require([
    "jquery",
    "mage/template",
    "Magento_Ui/js/modal/alert",
    "jquery/file-uploader",
    "Magento_Theme/js/sortable",
    "mage/translate"
], function ($, mageTemplate, alert) {

    $('#js_files_uploader').fileupload({
        dataType: 'json',
        replaceFileInput: false,
        sequentialUploads: true,
        url: '{$block->escapeJs($block->getJsUploadUrl())}',

        /**
         * Add data
         * @param e
         * @param data
         */
        add: function (e, data) {
            var progressTmpl = mageTemplate('#js-file-uploader-template'),
                fileSize,
                tmpl;

            $.each(data.files, function (index, file) {
                fileSize = typeof file.size == "undefined" ?
                    $.mage.__('We could not detect a size.') :
                    byteConvert(file.size);

                data.fileId = Math.random().toString(36).substr(2, 9);

                tmpl = progressTmpl({
                    data: {
                        name: file.name,
                        size: fileSize,
                        id: data.fileId
                    }
                });

                $(tmpl).appendTo('#js-file-uploader');
            });

            var uploadButton = $('#js_uploader_button');
            uploadButton.prop('disabled', false);

            uploadButton.on('click', function () {
                $('#messages').html('');
                $(this).attr('disabled', 'disabled');

                data.submit();
            });
        },

        /**
         * On done event
         * @param e
         * @param data
         */
        done: function (e, data) {
            $('#no-js-files-found').remove();

            var progressSelector = '#' + data.fileId + ' .progressbar-container .progressbar';
            $(progressSelector).css('width', '100%');

            $(this).val('');

            if (!data.result.error) {
                $(progressSelector).removeClass('upload-progress').addClass('upload-success');

                $('#' + data.fileId).delay(2000).fadeOut(2000);
                $('body').trigger('refreshJsList', {
                    jsList: data.result.files
                });
            } else {
                $(progressSelector).removeClass('upload-progress').addClass('upload-failure');
            }

            $('.ui-sortable').sortable('initButtons');
        },

        /**
         * On progress
         * @param e
         * @param data
         */
        progress: function (e, data) {
            var progress = parseInt(data.loaded / data.total * 100, 10);
            var progressSelector = '#' + data.fileId + ' .progressbar-container .progressbar';
            $(progressSelector).css('width', progress + '%');
        },

        /**
         * Fail event
         * @param e
         * @param data
         */
        fail: function (e, data) {
            var progressSelector = '#' + data.fileId + ' .progressbar-container .progressbar';
            $(progressSelector).removeClass('upload-progress').addClass('upload-failure');

            $(this).val('');
            alert({
                content: $.mage.__("We don't recognize this file extension.")
            });
        }
    });

    $('#js_files_uploader').on('click', function () {
        /** Unbind click event on file change */
        $('#js-file-uploader').html('');
        $('#js_uploader_button').off('click');
    });


});

script;
?>
<?= /* @noEscape */ $secureRenderer->renderTag('script', [], $scriptString, false) ?>