Submit a nested form via ajax to upload a file inside modal window does not work on IE.

I'm trying to submit a nested form via ajax to upload a file inside modal window.
I get an exception:
java.lang.IllegalStateException: ServletRequest does not contain multipart content. One possible solution is to explicitly call Form.setMultipart(true), Wicket tries its best to auto-detect multipart forms but there are certain situation where it cannot.
at org.apache.wicket.protocol.http.servlet.MultipartServletWebRequest.<init>(MultipartServletWebRequest.java:113)

After debug:
FileUploadBase.isMultipartContent get false because

if (contentType.toLowerCase().startsWith(MULTIPART)) - contentType = application/x-www-form-urlencoded

It only occurs on IE browser (both IE7 and IE8).