For now, this functionality is only available in Firefox and Chrome, but Google plans to roll it out for other browsers "as soon as they support this feature." And to make your life even easier, check out more quick and easy Gmail tips.
It's simple, it's speedy, and drag-and-drop functionality is, in my opinion, one of the best features any application can have. Now, finally, you can drag and drop attachments while using Gmail. Instead of clicking "Attach a file" and searching for attachments one by one, you can just drag the photo, document, or other attachment onto the message. It works with multiple files, too — so if you have a folder full of photos to share with a friend, just select all of them and drag them over to attach.