With Snow Leopard, Apple changed the way files are associated with applications. Previously, every file on a Mac had a hidden creator code which specified which application a file “belonged” to. Those codes date all the way back to the introduction of the Macintosh in 1984.

Files’ default applications are now globally associated based on their file extension.

To change the default application for a given file type, use the Finder to Get Info on any file with the extension you’re changing. Here’s an example of how to change all EPS documents to open in Illustrator:

In the File Info window, select your preferred application from the Open with popup menu. After selecting a new application, clicking the Change All button will make this the default application for all files with the same extension.