I wonder if a numeric in a file extension would cause problems in some systems or violates some convention.
I think 7zip uses 7z almost everywhere by default ?
.mp3 is pretty common
At least on Windows, bzip2 and 7z have been common archives for a while. Additionally, ps1 is used by Windows PowerShell. I expect the former two to cause no issues on common Unix distros either.
Oooh I may have to do this!
K7 also works in Portuguese, and possibly other languages.
I mean, while K exists in the Portuguese alphabet, no words use it.
W, K, and Y were added back to the Portuguese alphabet in 2009, after going away in 1943, exactly because there are words using them.