Form Plugin file upload broke after upgrading from jQ 1.3.2 to 1.4.2

Form Plugin file upload broke after upgrading from jQ 1.3.2 to 1.4.2

I have an ajax file upload form that was working fine with ajaxSubmit before I upgraded to 1.4.2. After upgrading, however, it broke: now the file is still uploaded (I see the successful POST in my server log), but the success callback does not fire.

The ajaxSubmit call has options {iframe: true, dataType: 'json'} and the server returns JSON wrapped in a textarea.

I was using version 2.36 of the plugin. I upgraded to 2.43 but this did not solve the problem.