How to get value from listbox in TinyMCE

I want to get the value in the listbox but it always return text value (Yes/No instead of True/False). This is my code:

onclick: function () {
    ed.windowManager.open({
        title: 'A Grid based list of most recent blog posts.',
        body: [
            {
                type: 'listbox',
                name: 'posttileBox',
                label: 'Display post title',
                'values': [
                    {text: 'Yes', value: 'true'},
                    {text: 'No', value: 'false'}
                ]
            }
        ],
        onsubmit: function( e ){
            ed.insertContent( '[recentblog title="' + e.data.posttileBox + '"]');
        }
    });
}

Thanks for any help.

Regards, Hung The.