success is not getting fired

[coup]

I have a following code but when my server returns success:true in json

success event is not getting fired

{

                        xtype: 'fileupload',
                        id:'noUpload',
                        width:100,
                        height:40,
                        margin:'0 10 10 10',
                        url: myurl,
                        success:function(){


                            alert('success');

                        },
                        failure:function(){

                            alert('fail');

                        }

                }