The new Gmail compose box added a very useful new feature to the service. You can now send mails with attachments in background. There is no need to wait for the attachments to upload while sending mails with large files.

Google shows the progress at the top and message is sent entirely in the background. Earlier you could send mails in the background but attachments needed to be uploaded in advance.

Should come handy for people who have to send attachments frequently through Gmail.