If your system is having trouble reading the file extension:

: VLC Media Player also has a built-in "Convert/Save" feature that lets you process video files locally.

: Right-click the cover art box, select "Add cover," choose your desired image file, and save the metadata to permanently embed the image into the MP4 file. 🔄 How to Convert the Video

If you need a quick visual guide on how to open or troubleshoot standard MP4 files on your computer, watch this tutorial: How to Create and Open an MP4 File YouTube• Nov 2, 2016