From Mahable.com: Gmail has a new feature for Firefox 3.6 and Google Chrome users: drag-and-drop file attachments. The feature is very straightforward — just drag files from your desktop onto your e-mail, and a green box will appear where you can drop your files.
Google promises it will “enable this for other browsers as soon as they support this feature.” Why the wait? Drag-and-drop functionality is an HTML5 feature. Currently only the Gecko layout engine — the engine that powers Firefox — fully supports HTML5 drag-and-drop. WebKit, which powers both Safari and Chrome, has only partial support for drag-and-drop.