In Mac OS X, when you doubleclick on a file name, that file will open with the application which created the file. So there must be stored, somewhere in the file description on a Mac OS X file, information on which application created this file. Note that this is not the file extension, the file can have any extension, or no extension at all.

Two questions regarding this information:

(1) How can I search for all files which will open a specific application?

(2) How can I see, and change, the application which a certain file will open?