jQuery Forum

media_array  - is collection of dom elements and not jQuery objects, attr() - is a jQuery object function
you need to "convert" media_array[0] first if you want to use attr()

$('.media').each(function() {
      alert( $(this).attr('íd') );
});

//or

alert( $(media_array[0]).attr("id") );

//or

alert(media_array[0].id);