GeoExt form and file upload

Question

I have a GeoExt form and want to do file upload here. But i never work with GeoEXT. I have this.

buttons: [{
  text: 'Загрузить',
  handler: function(){
  if(fp.getForm().isValid() && Ext.getCmp('opinions').getValue()=="blabla"){
    form_action=1;
    fp.getForm().submit({
             url: '<portlet: actionURL>',
             waitMsg: 'WAITING...',
     success: function(fp, o) {
              DO SOMETHING
              }
             }
            }
          }]

This is correct?
What arguments use in success: function(fp, o)? First argument is FormPanel but i dont understand what is second. Another question how to do asynchronous file upload in this case?