You can store metadata on any file in the mac filesystem(HFS+). If you want to ensure the file has a specific encoding, or if you want to place a checksum with the file you can use file attributes.
Setting and reading are easy on macOS with the
> touch something.txt > xattr -w xyz 123 something.txt > xattr -p xyz something.txt 123
In this example we wrote(
xattr -w) an attribute onto the file and printed
xattr -p) to see that it was written.
You can list the file attributes with a cryptic option for ls,
ls -l@. Try
doing this on your
~/Downloads dir to learn something cool about how macOS
treats files coming from the internet.