first upload all files

Modules/Product/Resources/assets/admin/js/Download.js

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+export default class {
+    constructor(download) {
+        this.downloadHtml = this.getDownloadHtml(download);
+    }
+
+    getDownloadHtml(download) {
+        let template = _.template($('#product-download-template').html());
+
+        return $(template(download));
+    }
+
+    render() {
+        this.attachEventListeners();
+
+        return this.downloadHtml;
+    }
+
+    attachEventListeners() {
+        this.downloadHtml.find('.delete-row').on('click', () => {
+            this.downloadHtml.remove();
+        });
+
+        this.downloadHtml.find('.btn-choose-file').on('click', () => {
+            let picker = new MediaPicker();
+
+            picker.on('select', (file) => {
+                this.downloadHtml.find('.download-name').val(file.filename);
+                this.downloadHtml.find('.download-file').val(file.id);
+            });
+        });
+    }
+}

Modules/Product/Resources/assets/admin/js/Downloads.js

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+import Download from './Download';
+
+export default class {
+    constructor() {
+        this.downloadsCount = 0;
+
+        this.addDownloads(FleetCart.data['product.downloads']);
+
+        if (this.downloadsCount === 0) {
+            this.addDownload();
+        }
+
+        this.attachEventListeners();
+        this.makeDownloadsSortable();
+    }
+
+    addDownloads(downloads) {
+        for (let attributes of downloads) {
+            this.addDownload(attributes);
+        }
+    }
+
+    addDownload(attributes = {}) {
+        let download = new Download({ download: attributes });
+
+        $('#downloads-wrapper').append(download.render());
+
+        this.downloadsCount++;
+        window.admin.tooltip();
+    }
+
+    attachEventListeners() {
+        $('#add-new-file').on('click', () => {
+            this.addDownload();
+        });
+    }
+
+    makeDownloadsSortable() {
+        Sortable.create(document.getElementById('downloads-wrapper'), {
+            handle: '.drag-icon',
+            animation: 150,
+        });
+    }
+}

Modules/Product/Resources/assets/admin/js/ProductForm.js

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+export default class {
+    constructor() {
+        this.managerStock();
+
+        window.admin.removeSubmitButtonOffsetOn([
+            '#images', '#downloads', '#attributes', '#options',
+            '#related_products', '#up_sells', '#cross_sells', '#reviews',
+        ]);
+
+        $('#product-create-form, #product-edit-form').on('submit', this.submit);
+    }
+
+    managerStock() {
+        $('#manage_stock').on('change', (e) => {
+            if (e.currentTarget.value === '1') {
+                $('#qty-field').removeClass('hide');
+            } else {
+                $('#qty-field').addClass('hide');
+            }
+        });
+    }
+
+    submit(e) {
+        e.preventDefault();
+
+        DataTable.removeLengthFields();
+
+        window.form.appendHiddenInputs('#product-create-form, #product-edit-form', 'up_sells', DataTable.getSelectedIds('#up_sells .table'));
+        window.form.appendHiddenInputs('#product-create-form, #product-edit-form', 'cross_sells', DataTable.getSelectedIds('#cross_sells .table'));
+        window.form.appendHiddenInputs('#product-create-form, #product-edit-form', 'related_products', DataTable.getSelectedIds('#related_products .table'));
+
+        e.currentTarget.submit();
+    }
+}

Modules/Product/Resources/assets/admin/js/main.js

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+import Downloads from './Downloads';
+import ProductForm from './ProductForm';
+
+new ProductForm();
+new Downloads();

Modules/Product/Resources/assets/admin/sass/main.scss

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+#images, #attachments {
+    margin-bottom: 15px;
+}
+
+.attachment-wrapper {
+    tr {
+        td {
+            &:nth-child(2),
+            &:nth-child(3) {
+                min-width: 200px;
+            }
+
+            &:nth-child(4) {
+                width: 59px;
+            }
+        }
+    }
+
+    .choose-file {
+        padding: 10px 15px;
+    }
+}