Although I don't receive messages with attached video files several times a day, occasionally one visits my inbox. Since I'm spoiled by other attachments to click straight to their preview and view them in all their glory, I do the same with videos. Even more annoying then is the fact that I have to first close the Gmail message telling me that the attachment cannot be viewed. Then I humbly save everything and only view it.
A couple of days ago, I decided that enough was enough and finally solved the problem by installing a very simple web browser extension.
All you have to do is install Gmail Mod browser extension:
After installing the extension, just restart your web browser and try it out on the first video file message you see in Gmail.
You'll see a new icon next to the attachment tile with the Play arrow symbol. Just click on it and the video will start playing right away in your web browser. In the case of Firefox, it's in a new window, which can be easily closed by pressing the Esc key when you're done playing.
This extension will also offer fast playback of the included audio.