Ext Js uploading file brings up one file dialog on response

Question

I have the following code which I am submitting to an aspx page

 fp.getForm().submit({
                         url: 'SomeAspxPage',
                         method: 'POST',
                         waitMsg: 'Uploading your file...',
                         isUpload: true, 
                            success: function(fp, o){
                               alert('success');                                
                         },
                         failure: function(form, action) {
                                alert('failed)';
                            },
                            form: fp.getForm().getEl().dom
                        });

The files get submitted and I can save it ot do whatever. My problem is that even though I set the response content-type to be application/json the browser always opens the 'open file' dialog which contains the json string I am returning. Does anyonw know why this might be?