Today Google introduced a new feature to the Gmail users. Now the users are able to work on other parts of the app while the email is being sent in the background.
Google pointed out in their announcements that as the email sending can take several seconds depending upon the data being sent, the heavy gmail users might find it irritating sometimes. Background Send dumps you back into your gmail inbox while the process of sending the email completes behind the scenes.
If you have just attached the file before hitting send, the upload will be completed and then the message will be sent however there maybe some bugs with it, so test it out before you are sure it works for you all the time (specially on your browser)
Hopefully Google will fix the background upload and send for the Background send for gmail too!