jquery.forms non-contained input bug

HTML

<script type="text/javascript" src="http://malsup.github.com/jquery.form.js"></script>


<form id="a">
  <input type="text" name="aa" value="aaa">

  <input type="text" name="bb" value="bbb" form="b">
  <input type="file" name="file" form="b">
  <input type="submit" name="upload" form="b" value="Upload">
</form>


<form id="b">
</form>

JavaScript

$("#a").ajaxForm({
    beforeSubmit: function(arr, $form, options){
        for (it in arr) {
            alert(arr[it].name); 
        }
        return false;
    }
});