Sites like twittervideodownloader.com or twdown.net (use caution with ads).
Here is a complete overview of what this file likely represents, how to handle it, and how to create/manage such files: 1. What is a "Twitter (10) mp4" file? Twitter (10) mp4
Tools like CloudConvert or Ezgif can convert .mp4 to other formats (e.g., GIF, MOV). Sites like twittervideodownloader
Use the native photo gallery/video app on your iOS or Android device. 3. How to Convert or Edit (If Needed) If the file is not playing properly or you need to edit it: how to handle it