how to select an element inside FormType symfony?

this works fine

  $(document).ready(function () {        $('#banner_description').on('keyup', function () {            alert( this.value );        })    });

and when i need to catch the change event when i do upload images its doesn't work i would like to allow to upload just 4 images there is any solution ?

    $('#banner_images').on('upload', function () {        alert( this.value.length );    })

Form Type inspecct