OS X has more than one way of associating files with apps, and it stores the data in different places. It supports OS 9-style type/creator codes, which are part of the file's HFS metadata, and thus don't transfer well to non-HFS-formatted disks (for some reason Apple does not like to use this with their new apps). It also supports default apps based on file name extensions, file-specific association, and type-specific associations (which considers both HFS creator codes and extensions), all of which can be manipulated from the Get Info window. This data (or at least part of it) is stored in ~/Library/Preferences/com.apple.LaunchServices.plist