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.
If you are experiencing issues displaying video files in 5.10.x or later, you may still have your attachments stored in the database, rather than on disk. See the following KB for further details.