Gmail looks in the eyes of internet users, especially email owners, and said, "Uploading attachments are less tedious and easy to estimate if you're using ME instead of other email service providers."
Gmail now provides multi-select and progress bars to help their users when they're attaching more than 1 file.
Some people might not consider this additional feature as something important, but this is actually parts of the principles in GUI (Graphical User Interface) design. The multi-select feature is about making even the simplest task simpler to do; that is uploading multiple files. The progress bars provides better feedback for users so that they can easily estimate how much longer will it take for the upload to finish.
I'm glad that the Gmail team never stopped enhancing Gmail. Maybe that's one of the reason why they keep the word "beta" close to the Gmail logo. That means they'll never forget that what they have right now is just the beta version and requires more and more improvements.
What about you, Yahoo?