It seems Gmail is just starting this feature, and it doesn't require you to install any plugin etc.
It works in both Firefox and Chrome but not IE.
Use dropzonejs.
It's a lightweight library supporting file drops by simply adding the dropzone class to form elements. It handles the files with HTML5 and even shows previews of images dropped into it. In incompatible browsers it falls back to a simple file upload form. Also: it looks good.
dropzone
Have a look at it!
Disclaimer: I wrote this library.
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