File magic

File magic refers to data that makes it easy to identify what kind of file it is.

Sometimes this is data that is intentionally there, for that purpose alone.

file magic also refers to tools that tell you file types that way, e.g. the file tool on *nix.

Which is sometimes less about seeing known data, and more about known structure - starting to parse that file, and seeing whether it keeps making sense. How much you have to parse before you can be sure will vary - usually not a lot, but it does avoid some false positives.