You can add one or more pieces of digital content as "Extras" and then include them for sale in a "Package".
(You can also create a package that just sells multiple copies of your book: http://help.leanpub.com/en/articles/3287107-can-i-make-a-package-to-sell-multiple-copies-of-a-book).
First, you should upload the content you want to sell in your package. To do this, in the Leanpub menu, go to:
Author > Books > [Your Book Title] > Store > Extras
Once your content is uploaded, you can add it to a package and sell it with your book. To do this, go here:
Author > Books > [Your Book Title] > Store > Packages
Then, you can add each extra you want to any package you create. The package will then be displayed for sale on your book's landing page, as an option under the pricing sliders at the top right. You can make multiple packages and sell them at different price points.
Here's an example of a book that also sells videos and other content in differently priced packages: https://leanpub.com/rprogramming.