WordPress is a great CMS and has a nice feature for uploading files. That is the media uploader which can be found inside the WordPress administration. In this tutorial we will not use the media uploader. We will develop our own WordPress file upload using AJAX and PHP.
Creating a button or a link to download a file from WordPress in text is pretty simple: add an image and point it to an URL of the file. In this tutorial you will learn how to programmatically download a file.
One of the problems in a WordPress Multisite is that you need to find a way to share media across all sites in the Multisite so that you can save space on your server.
Media sharing is already available with a plugin, but featured image is a different story.