// tell Mithril that data changed and a re-render is needed
            })
        }
    })
}</code></pre>
<p>In the example above, a file input is rendered. If the user picks a file, an upload is initiated, and in the <code>config</code> callback, a <code>progress</code> event handler is registered. This event handler is fired whenever there&#39;s a progress update in the XMLHttpRequest. Because the XMLHttpRequest&#39;s progress event is not directly handled by Mithril&#39;s virtual DOM engine, <code>m.red