The file extension is part of the file name, and it may be hidden by the Finder. The "Advanced" tab in the Finder preferences permits you to force the file extension part of the file name to be displayed. Otherwise the default is to hide the extension part of the file name unless you've explicitly asked Finder to display the extension for that particular file (select file, press command-I, and uncheck "Hide extension").
OS X still stores a "Creator Code" and file type for each file in the system metadata (as described in my previous post). You can see these using Terminal, and the command 'mdls'. OS X 10.6 uses this information differently than previous versions. OS X 10.6 maintains the creator code, but now ignores it in deciding which application to use to open a file.
The Finder (and the shell's "open" command) use the following criteria to decide what application to open a document with (stopping at the first one that fits):
1. Did the user specify the application using the Finder? If so, use that.
2. If the file has an extension in the name ('.' followed by a string that doesn't contain a '.'), and that extension is registered with LaunchServices, use the file type and default application registered with LaunchServices.
3. If the file meta-data contains a 4-letter file type (kMDItemFSTypeCode) and that's registered with LaunchServices, use the default application that's registered.
If the file type is selected by 2 or 3 and there are multiple applications that can work with those file types and the user hasn't specified a default, then check if the file has a creator code and use that as the default application (note that 10.6 no longer considers the creator code). If there's no creator match, give preference to OS X apps over Classic ones (10.6 no longer supports Classic apps). Give preference to apps on the boot volume over other volumes, and if there's two apps with different versions, give preference to the most recent.