.json is roughly the same as .txt - an abbreviated version of "JavaScript Object Notation". We humans still have to do the translation to the final name.
And, I've never seen ".jsn". I've seen ".json" hundreds of thousands of times, but never ".jsn".
>Computers should work for us. We shouldn't work for computers.
Dump .txt for .text.
If my computer expects me to type an extra letter every time I name a file it isn't working for me. The short extensions make for more efficient typing.
This is the first time I've heard of a jsn file. In my experience as a developer, they're always named with the full four letters. Also I really don't understand your statement about computers working for us, they're perfectly happy to interpret an extension anyway you set it up in the vast majority of file association systems of any modern day OS.
I don't know why you're getting hung up on TXT in particular, that extension has been around for so long that virtually everyone who has even a passing knowledge of computers knows what it stands for.
Also for CLI purposes, I'd sure as hell rather type "ls .bmp" than "ls .bitmap".
Dump .txt for .text.
Dump .jsn for .json.
Keep .html, .jpeg, etc because they are abbreviations for standards.
Sucks for Windows that it can't handle ".text" like other operating systems have since the 1980's.
Dumping three-letter extensions will also help avoid all the extension namespace collisions that happen all the time.