MP4 files not showing in Spotify

[Ksorek]

Plan

Premium

Country

 Poland

Device

PC

Operating System

Windows 10

My Question or Issue

 I can not add mp4 files to my Spotify playlists (they do not appear in local files).

I have installed QuickTime already and it does not help.

Hope you can help me, also I'd like to point out that I am adding those files to Spotify because I want to import them to my Spotify app for Samsung Galaxy S8+.