You can now test video and audio with Preference tests 🎉 - UsabilityHub updates
What Is an MP4 File (and How Do I Open One)?
Big list of sample videos for testers - standaloneinstaller.com
Sample MP4 files download | Video File Examples Download
Need Help: Unable to upload MP4 videos on web or app, got error message: “Your video file is not compatible. Please see tips for uploading videos.” - Media APIs - Twitter Developers