Attaching files to your post is quite simple. First bring up the new post window:
At the bottom left you will see controls for attaching images and other file types. Select either of these options and locate your file. Using this method you can attach as many files as you'd like, albeit one at a time.
Attached images will be displayed at the bottom of your post (you can also include inline images), and files of other types will be presented for download.