At the beginning of this month Gmail started supporting attachments of up to 50MB at long last, and today's feature addition to the web-based client probably has something to do with that.
Rolling out in the next 15 days to all Gmail users is a video streaming capability for attachments. This means you will no longer be required to download a video attachment in order to watch it.
Once this reaches you, video attachments will show a thumbnail from the clip. When you double-click on that, the content will just start streaming inside a video player that's very similar to the one YouTube uses. This lets you adjust the playback speed and videos are delivered by default at optimal quality. It's akin to what happens in Google Drive when you double-click on a video file. In fact, powering the new feature is the same infrastructure that's behind YouTube and Drive's streaming functionality.