Adding a Media Button to the WordPress Editor.

admin.js

jQuery(function($) {
  $(document).ready(function(){
    $('#insert-my-media').click(open_media_window);
  });

	function open_media_window() {
		if (this.window === undefined) {
			this.window = wp.media({
                title: 'Insert a media',
                library: {type: 'image'},
                multiple: false,
                button: {text: 'Insert'}
            });

			var self = this;
			this.window.on('select', function() {
                var first = self.window.state().get('selection').first().toJSON();
                wp.media.editor.insert('[myshortcode id="' + first.id + '"]');
            });
		}

		this.window.open();
		return false;
	}
});

init.php

<?php
// Add Btn after 'Media'
add_action( 'media_buttons', 'add_my_media_button', 15);
function add_my_media_button() {
	echo '<a href="#" id="insert-my-media" class="button">Add my media</a>';
}

// Add js to admin
add_action( 'wp_enqueue_media', 'include_media_button_js_file' );
function include_media_button_js_file() {
	wp_enqueue_script( 'media_button', plugin_dir_url( __FILE__ ) . 'assets/admin.js', array('jquery'), '1.0', true );
}


// Inserts new tab to wp.media
add_filter( 'media_upload_tabs', 'my_media_menu' );
function my_media_menu( $tabs ) {
	$newtab = array( 'my_custom_tab' => __('My Tab') );
	return array_merge($tabs, $newtab);
}

If I set multiple to true, how do I get the second id?