When Wicket is handling multipart form sent via ajax call it does not think that it is an ajax request. The problem is in the fact that HTTP POST generated by submitting inline frame does not contain Wicket-Ajax" header attribute.
As the result the resource's url (particularly when the urs is being prepended) does not contain the sequence of "../" characters. This in turn results into 404 response on the browser side and the page is not updated and isn't in sync with the wicket server state anymore. Hence the WICKET-2491 issue. I believe the problem would be reproduced with any mounted page. Even links to css cause the similar problem - instead of 404 I obtain "the resource is moved" warning. Having struggled with the problem about a day (I would like the fix to be part of 1.4.2) I found the cause and came up with a patch. See https://issues.apache.org/jira/browse/WICKET-2491 Igor please have a look.
