Question

I have a file which was originally named foo in the Finder. If I get info for the file, the Finder recognizes it as "Kind: Plain text". If I rename the file as foo.png , Finder then recognizes it as "Kind: Portable Network Graphics image". If I rename the file as foo , Finder still recognizes it as "Kind: Portable Networks Graphics image" (not plain text like I expected).

How is Mac OS X recognizing the file kind?

Answer 1

Magic numbers in the file is usually how it works. A PNG file, for example, has a header to identify it as PNG and not some other file format. Wikipedia has some more info.

Answer 2

I don t know how it works, but I can imagine that until you renamed the file to foo.png Finder didn t know and didn t care what type it is. But after you ve renamed it Finder stored it s type somewhere (.DS_Store perhaps?) with the inode ID so it could recognize this file even after you rename it again.

Answer 3

Because *NIX systems (like Mac OS X) don t rely on extensions to recognize file types. They rely on something called Magic Number (if I remember well), that is, they search for common patterns found in those types of file (e.g headers, tags, metainfo etc.) and they recognize them.

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