Hi @usc1787,
Thanks for your feedback,
Can you share which mobile app you are using? Is it the classic app downloaded from the app store / play store?
I’ve tried to reproduce your issue on our nightly build server running off the latest master using the Mattermost app (not classic) from the app store and can confirm that an mp4 video plays for me on my iOS device. (After uploading a video file, you will first need to click on “download” to download it to your device and once it’s downloaded the “download” changes to a “play” button)