Hi,
Thanks for getting in touch with us.
SKYBOX does support Plex media files.
You might want to try downloading some third party apps to see if you can see all the media files there, like VLC.
If you can't see the media files with VLC either, you might have to turn on some settings in your Plex server. Otherwise this might have something to do with the current DLNA protocol we are using, which will be changed/upgraded in the next couple of months.
We appreciate your patience :)
Happy new year
Andy