wp.media.view.Toolbar.Custom = wp.media.view.Toolbar.extend({
    initialize: function() {
        _.defaults( this.options, {
            event: 'custom_event',
            close: false,
            items: {
                custom_event: {
                    text: wp.media.view.l10n.customButton,
                    style: 'primary',
                    priority: 80,
                    requires: false,
                    click: this.addAttachment
                }
            }
        });

        wp.media.view.Toolbar.prototype.initialize.apply( this, arguments );
    },

    // triggered when the button is clicked
    addAttachment: function(){
        this.controller.state().addAttachment();
        this.controller.setState( 'insert' );
        // I NEED TO TRIGGER A REFRESH OF THE ATTACHMENTS TAB HERE
    }
});