jquery.form plugin IE7 windows nagivating away on response

jquery.form plugin IE7 windows nagivating away on response

My goal is to be able to submit a form including a file input in a ajax manner and the jQuery Form Plugin seems suitable to achieve this. Hover my requirement is that it should also work in IE7. I've created a small prototype to test out the the functionality and the form including the file is submitted and received properly by the backend. I've already changed the way json is returned (as text) to prevent IE from showing a save file dialog.
In IE7 when the response is received from the backend (either a 200 or 400), the browser navigates the window away.

The view looks like:
  1. <form action="" method="post" name="my-form" id="my-form"  enctype="multipart/form-data">
  2.     <input type="text" name="Message"/>
  3.     <input type="file" name="File"/>
  4.     <input type="submit" value="Send">
  5. </form>

  6. <script type="text/javascript">      

  7.         $(function () {
  8.             var options = {
  9.                 clearForm: true,
  10.                 iframe: true,
  11.                 type: "POST"
  12.             };

  13.             $("#my-form").ajaxForm(options);
  15.             $("#my-form").on(
  16.                 "submit",
  17.                 function (args) {
  18.                     args.preventDefault();
  20.                     $(this).ajaxSubmit();

  21.                     return false;
  22.                 }
  23.             );
  24.         });
  25.     </script>
The backend (MVC) looks like (just a simple 200 with "Ok"): 
  1. return base.Content(
  2.                     string.Format(
  3.                         "<textarea>{0}</textarea>",
  4.                         Newtonsoft.Json.JsonConvert.SerializeObject("Ok")
  5.                     ),
  6.                     "text/html"
  7.                 );
Does anyone have a solution to prevent IE7 from navigating the window away? It would be highly appreciated since it seems to be the only issue which prevents me from achieving my goal.