Most modern media players place a heavy focus on video, album art, libraries, digital rights management (DRM), thumbnails, and metadata. NanoStereo just plays the audio.
Every installation of NanoStereo plays MP3, WMA, MIDI, WAV, and CD Audio.
If you can't play certain file types (M4A, AAC, FLAC, OGG, OPUS) and would like to add that ability to NanoStereo, install a codec pack such as the K-Lite Codec Pack that includes an audio decoder for those formats.
M3U is a plain text file. Create/Edit M3U with Notepad or any plain text editor. Each line is an entry in the playlist and can be a local file path or web address.