If you've ever received a video attachment in Gmail, the only option was to download the file to view it. That's changing today, as Google is now rolling out the option to stream video attachments from the Gmail desktop client.
The feature relies on the same infrastructure as that powering YouTube and Google Drive previews, allowing Google to deliver the video at "optimal quality and availability." When you click on a video thumbnail, you'll now see a YouTube-style player in the window, with options to adjust playback speed and audio levels. However, the feature doesn't seem to work just yet, as I got the following error when I tried playing a video:
Gmail's attachment limits apply to videos as well, which means that individual file sizes are still restricted to 25MB (50MB if you're sending from another mail provider). That said, the ability to stream videos directly from the browser makes it more convenient to view video attachments. Google is gradually rolling out the feature, with all users expected to receive it within 15 days.