$.attr doesn't work, in on click handler it keeps attribute old value after overriding its value

[pajoe@…]

$(".imgBox").append("<input id='tt' type='checkbox' />");
$(".imgBox input").click(function(){
    if ($(this).attr("checked"))
        $(this).attr("checked", "checked");
    else
        $(this).attr("checked", null);
});

[dmethvin]

Duplicate of #10434.