The procedure involves dragging a file from hard drive or any open application into a green box within the compose menu, the service then uploads it in the message background. It tends to follow the same pattern as the system implemented in Google Wave service for images and other medias.
The drag and drop attachments feature, however, is restricted to users running Google Chrome and Mozilla Firefox for now, with a later extensions to other browsers as soon as they can support the feature.
Mozilla Firefox users with pre-installed Mozilla Prism who are running Gmail as a stand-alone desktop app gets a whole new functionality, serving as a whole new software. The feature makes file attachments easier and more integrated into the Gmail calendar service.