Adds a button to WordPress 3.5 Backbone-powered uploader/media library modal.

gistfile1.js

var wpMediaFramePost = wp.media.view.MediaFrame.Post;
wp.media.view.MediaFrame.Post = wpMediaFramePost.extend(
{
	mainInsertToolbar: function( view )
	{
		"use strict";

		// first of all, call the original functions from prototype
		wpMediaFramePost.prototype.mainInsertToolbar.call(this, view);

		var controller = this;

		this.selectionStatusToolbar( view );

		view.set( "button-name", {
			text: "button text",
			style: "primary", // button class
			priority: 80,

			requires: {
				// requires an active selection of attachments for the button to be enabled
				selection: true
			},

			// function which will be executed when you click the button
			click: function()
			{
				var state = controller.state(),
				selection = state.get("selection"); 
			}
		});
	}
});

inspired by https://gist.github.com/Fab1en/4586865