[idx]

hrm.. works for me. eg:

<script>
<!--
  function check_form(form) {

     if (form.foo.value == "") {
        alert("foo is required!");
        form.foo.focus();
        return false;
     }

     return true;
  }

//-->
</script>

<form name="me" action="<?=$_SERVER['PHP_SELF']?>" method="POST" onsubmit="return check_form(this);">

<input type="file" name="foo">
<input type="submit">
</form>