在返回的HTML中上传文件返回HTML标签的iframe方法

I am using the iframe jquery method to upload a file to my server. My PHP file that processes the uploda returns a JSON string. The returned data is coming back with HTML in the data (which I imagine Firefox is putting in). How can I stop this or just strip out the JSON?

function uploadFile() {
      $("#invoice_upload_form").submit();                                                                                        
      $('#invoice_upload_frame').on('load',function(e) {
        e.preventDefault();                   
        if($(this).contents()[0].location.href.match($(this).parent('form').attr('action')))
        {
          var html_return = $(this).contents().find('html').html();
          //var json_return = JSON.parse(html_return);
          alert(html_return);
          //alert(json_return);                                        
        }
      });
    }

Comes back as:

<head><link rel="alternate stylesheet" type="text/css" href="resource://gre-resources/plaintext.css" title="Wrap Long Lines"></head><body><pre>{"success":false,"message":"Error importing invoice file"}</pre></body>