ใช้ HTML5 ซึ่งเวลานี้ support เฉพาะ FF และ Chrome ครับ
Google continues to use HTML5 to push its web apps into the future. The latest bit of HTML5 to feel Google’s love is drag-and-drop support, which is now a standard part of Gmail. If you’re using Google Chrome 4 or Firefox 3.6, you can now simply drag a file from your desktop onto a message window and Gmail will automatically attach the file.