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.
And, I've never seen ".jsn". I've seen ".json" hundreds of thousands of times, but never ".jsn".
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.
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".