<html>
<head>
  <title>Page with POST form</title>

<script>
  function SubmitForm() {
    document.forms['doit'].submit();
  }
</script>

</head>
<body>
  <form id="doit" action="/downloads/image.jpg?allow-post-only" method="post">
    <input type="Submit" value="Do It"/>
  </form>
</body>
</html>