An OSU file is a game script written for osu! rhythm game. It contains information about a beatmap used in the game such as the difficulty level, the song to be played, and the timings of the hit objects to which the player has to sync with the music. It lets rhythm game players to develop custom challenges and share with the game community.
MIME Type of OSR File Format: x-osu-beatmap
OSU File Format
OSU files are saved as plain text files to disc and its structure is very clearly defined in the OSU file format by osu!.