Passing value of a variable to angularjs directive template function

I am trying to pass a $scope's variable to a directive, but its not working. I am catching the variable in the template function:

app.directive('customdir', function () {

    return {
        restrict: 'E',

        template: function(element, attrs) {
            console.log(attrs.filterby);
            switch (attrs.filterby) {
                case 'World':
                    return '<input type="checkbox">';
            }
            return '<input type="text" />';
        }
    };
});

What I need is the value of variable filterby not the variable name itself.

Plunkr Demo