Why does a video file that appears to be an MP4 not play in Windows Media Player? I have extracted the files from an iOS app for a video streaming site that allows you to download videos (only on the mobile app, so I had to jailbreak to get the files out), but the file extension on the video file is something completely arbitrary specific to the app. If I just open it with MPC-HC or VLC it plays just fine and according to MediaInfo both the video and audio are in mp4 format (AVC/AAC respectively). However, if I just rename the file to .mp4, Windows Media Player refuses to play it so I'm guessing that means that wasn't the original file format? Based on the service that uses the file, I'm assuming that it must have been either MP4 or FLV or something along those lines.

Of course, I can just open the video in avidemux and just re-encode it to an mp4 if I really want to but I'd like to avoid losses in quality if possible.
Probably a fragmented MP4.  Remux it using ffmpeg.

ffmpeg -i filename -c copy out.mp4