Return filename for image field

Return the filename of the image form field instead of ``undefined``
when the File API is not available.  This lets us differentiate between
the case where no value has been specified for the field and when the
File API isn't supported.

Addresses #595
Support IFRAME syncs in Internet Explorer

Update ``MultipartFileUploadModelResource`` so passing an ``iframe``
querystring argument causes the response to be wrapped in a ``textarea``
field and the content-type set to ``text/html`` to use the jQuery Iframe
Transport's trick for detecting the response data type.

Addresses #595