I thought when you make a file on OS X it doesn't include the extension by default, but there's sometimes an option to include it, presumably for Windows compatibility for the same reason as downloaded files always have extension - if a Windows user downloads the file, it will only be recognised by Windows if it has an extension. Maybe Macs use the resource fork, but can also use the extensions as a fallback, for instance if someone sends you a file from Windows that of course has an extension but no resource fork? If you rename the file to remove the extension, it still recognises the type I think.