Reading external file after build ( .mp4)

So I want to make an VR media player where you can add mp4 files after the game is build. When I start the application I want to get all mp4 files to get read and added to an arraylist.

Is it even possible to add those mp4 files after the game is already build?

Sorry if my grammar is wrong english isnt my first language.. and if there is already a post about this could give me the link? I didnt find anything by google except for adding text files.

Thank you very much