.7z — 7-Zip Archive
Archive application/x-7z-compressed
What a .7z file is
The 7-Zip format, which usually compresses noticeably better than ZIP thanks to LZMA2.
How to open it
7-Zip on Windows, Keka on macOS, p7zip on Linux.
Worth knowing
Better compression, but less universally supported. Use ZIP when you do not know what the recipient has.