Mac OS X Panther in a Nutshell, 2nd Edition

File Types

A file is a basic unit of filesystem currency in any modern operating system. The following sections discusses some of the file types you’ll find in Mac OS X.

Mac OS X’s Application-to-Document Map

The system determines a document’s kind in one of two ways. First it sees if the document has an attribute fork, a data attachment possessed by documents created by Classic and Carbon applications that provides information about the document’s type (among other things). If the file lacks a attribute fork, it looks to the document’s filename extension; Mac OS X maintains a system- wide map between these extensions and recognized document types.

The system’s map that binds particular filename extensions to certain Aqua applications is made from two sorts of files. Each application’s Info.plist file (see Chapter 22) can define the filename extensions its documents use. For example, Terminal application files have the extension .term, and it says as much in its Info file (located, for the curious, at /Applications/Terminal.app/ Contents/Info.plist. If only one application lays claim to a particular file extension (as is the case with .term, at least in a fresh Mac OS X installation), the system will recognize a binding between that application and all files with that extension; double-clicking these documents in the Finder will open them through that application.

These claims, however, may be overruled by the contents of another file: com.apple.LaunchServices.plist, which is another ...