The new File Preview does not support viewing video attachments e.g mp4, mov. Upon clicking on a link to video attachment, the dialog shows:
We can't preview this file.
You'll have to download the file to view it.
At least, the feature should allow playing videos in the browser.
Previously, clicking on a link to video attachment will play the video in the browser (handled via browser plug-in). The new behaviour forces the user to download the video file to view it.
Alternative to this feature - use the Multimedia macro.