Edit in JSFiddle

function insertForm() {
    var form_body = '<form id="form2" action="action2">\
      <input type="file" name="filename" /><br />\
      <input type="submit" value="Загрузить" /><br />\
    </form>'
    var box = document.getElementById("box");
    box.innerHTML=form_body;
}

insertForm();

<div>
    <form id="form1" action="action1">
        <input type="submit" value="выход" />
    </form>
</div>
<br />
<div id="box"></div>