Like when you send a .7z instead of a .zip or .rar to a friend or a teacher because that's what your computer has installed and they're like "Oh No, not one of those, now I have to install 7Zip" even though the same program that opens .rar also opens .7z I feel like people are way more annoyed when they receive a .7z
It's common when you "wrap" one file type inside another. Like .tar combines multiple files into one, then .gz compresses a single file.
You also see it with PGP (encryption).
Suppose I have a javascript file for a node server's backend access named
db.jsSuppose I write tests for those functions and name the test script file
db.test.jsSuppose I tar and gzip that file (bear with me), now named
db.test.js.tar.gzSuppose I sign that file with PGP, now named
db.test.js.tar.gz.pgpNow suppose I want to hide that signed compressed tarball of a javascript tests file for my db functions, and to do so, I name it
.db.test.js.tar.gz.pgpNow I have a file that looks like it consists of nothing but extensions. I'm sure you could push it even further though, if you tried.