Open .m4s File Guide

If your M4S files are from a Steam game recording, you can use the built-in Steam recording and screenshots tool to export them as a standard MP4. 2. How to Open M4S Files on Mobile (Android & iOS)

For mobile devices, you generally need third-party apps that support adaptive streaming formats: How to play .m4s file given in mpd of MPEG-DASH on player? open .m4s file

It does include the initial ftyp or the full moov (those are in the initialization segment, often .mp4 or .m4i ). If your M4S files are from a Steam