How to Assign a File Type to an Application in Mac OS X Lion
Sometimes, if your Mac doesn’t recognize a file type as something that one of its programs can handle, or if you want a specific file type to always open in a specific program, Mac OS X Lion might need a little help from you. Mac OS X Lion lets you specify the application in which you want to open a document in the future when you double-click it. Suppose that you want all .tif graphic files that usually open in Preview to open instead in Pixelmator, a more capable third-party program
More than that, you can specify that you want all documents of that type to open with the specified application. “Where is this magic bullet hidden?” you ask. Right there in the file’s Info window.
Here’s how you do it:
Click one of the files in the Finder.
Choose File→Get Info (Command+I).
In the Info window, click the gray triangle to disclose the Open With pane.
From the pop-up menu, choose an application that Mac OS X believes will open this document type.
(Optional) If you click the Change All button at the bottom of the Open With pane, you make Pixelmator the new default application for all .tif files that would otherwise be opened in Preview.
Notice the handy alert that appears when you click the Change All button and how nicely it explains what will happen if you click Continue.