This is part of the version 2.18.90 update so if you haven’t updated already, you should if you want to check the feature out. According to MacRumors, it also appears that users who have disabled the media auto-download feature will also be able to download the media file directly from the notification without having to launch the app, which could come in handy if you’re busy doing something else.
That being said for those concerned about privacy, maybe you might want to disable it. It has also been pointed out that according to WABetaInfo, the feature doesn’t seem to be working for everyone just yet. WhatsApp is aware of the problem and will be fixing it so it should be up and running in due time. Note that this update is for iPhones that are running iOS 10 or later, so if you’re on an older build you will miss out on this.