numpy's npz is also zip.

Just look for that PK in the first 2 bytes of the file and it is a good chance it is a zip file. That jar/war one has saved me a few times in figuring out what exactly the compiler did to a program.

Yeah, it is surprising at first, but after you think about it, maybe not so much.

If you need to cram a bunch of files into one package, zip is the obvious candidate. There are well-tested libraries and apps for dealing with zips for essentially every language and operating system.

As the saying goes, "don't mess with success".