I've written this application as 1 file and included it with the application (dragdrop.txt). There are comments within the doc that provide some instruction

Normally there would be 3 files (makes it easier to edit and I reuse a lot of the style sheets and javascript functions for other apps)

You can even add an image

See my other JavaScript Example on Advanced DOM for code to alter this app further (hide or show, change or add to the text in boxes)