File Explorer won't thumbnail .MP4 files but will thumbnail .M4V files, why?

[TheBigFatClown]

It's a small thing that really bugs me about Windows 10 File Explorer. It will only thumbnail my .MP4 video clips if I rename the extensions to .M4V. I've read that .M4V is a proprietary Apple extension which suggests encryption. Yet, it works on my system. What's going on and is there an easier and more proper way to get the Windows 10 File Explorer to thumbnail my .MP4 files without changing the extensions?
Thanks for reading!