.flac — Free Lossless Audio Codec
Audio audio/flac
What a .flac file is
Lossless audio compression — bit-for-bit identical to the source at roughly half the size of WAV.
How to open it
VLC, foobar2000, most modern players. Apple added support in 2017.
Worth knowing
Use it for archiving a CD collection. For listening on a phone, a high-bitrate lossy file is indistinguishable and far smaller.