follow JQuery convention + browser compatibility

web_widget_image_download/static/src/js/web_widget_image_download.js

@@ -6,10 +6,10 @@ openerp.web_widget_image_download = function (instance) {
     'use strict';
 
     instance.web.form.web_widget_image_download = instance.web.form.FieldBinaryImage.include({
-        render_value() {
+        render_value: function () {
             this._super();
 
-            const widget = this.$el.find('.oe_form_binary_file_download');
+            var $widget = this.$el.find('.oe_form_binary_file_download');
 
             this.imgSrc = this.$el.find('img[name="image"]').attr('src');
 
@@ -18,7 +18,7 @@ openerp.web_widget_image_download = function (instance) {
                 url: this.imgSrc,
                 complete(xhr) {
                     // retrieve image type from server ("Content-Type" header)
-                    widget.attr('download', xhr.getResponseHeader("Content-Type").replace('/', '.'));
+                    $widget.attr('download', xhr.getResponseHeader("Content-Type").replace('/', '.'));
                 }
             });
 
@@ -27,9 +27,9 @@ openerp.web_widget_image_download = function (instance) {
                 this.$el.find('.oe_form_binary_file_clear').removeClass('col-md-offset-5');
             }
 
-            widget.attr('href', this.imgSrc);
+            $widget.attr('href', this.imgSrc);
         },
-        has_custom_image() {
+        has_custom_image: function () {
             // check if the image of the widget is different from the default placeholder
             return this.imgSrc && !this.imgSrc.includes('/placeholder.png');
         }