Movie files in Content/Movies stay outside the pak — the two fields that change it
By default, staging copies files from Content/Movies outside the pak as NonUFS, which is why .mp4 files sit loose in the build directory. Turning on bSkipMovies and listing the file under UFSMovies pulls it inside, and playback through UMediaPlayer with FileMediaSource keeps working: WmfMedia on Windows reads through IFileManager, which can access files inside the pak.