Missing data from a Jquery AJAXForm call in IE

Background: I'm using ASP.NET MVC 2.0 (and JQuery)

What's happening: I have a simple form

<form id="form1" action="Process/Review" method="POST" enctype="multipart/form-data">
<label> Upload File </label> 
<input type="file" name="file" id="file" />
<input type="submit" value="upload" id="uploadbutton">
</form>
<div id="reviewcontainer" />

in a fairly standard page. The javascript

$("$form1").ajaxForm({
    iframe: true,
    dataType: "html",
    success: function (result) {
        $("#reviewcontainer").html(result);
        return false;
    }
});

is inside a document.ready. The result is a partial form (.ascx) which contains another form very similar to the form above.

It works perfectly fine in Firefox, but in IE 8, the ret开发者_如何学JAVAurned form doesn't work. When I look at the "result" in the debugger, the form tags were gone.

Is there some obscure security feature in IE that doesn't like form tags being created by AJAX, or is it some other odd problem?