[Editor’s note: there are two similar but different FileTypes dialogs in BeOS. One (the FileTypes panel found in your Preferences folder) changes associations and settings for system-wide preferences, while the other (the FileType panel invoked when you right-click a file and choose FileType from the context menu) changes preferences for the current file only. The following example uses BeMail as an example, but the principle applies to any FileType on your system.]
Suppose you want to change the application that’s invoked when you double-click on a mail file. Normally, BeMail will be launched, but you want NiftyApp to be launched instead.
What you want to do is set the preferred application for a specific mimetype, in this case text/E-mail.
The steps to do this for PR2 are:

• open /boot/beos/preferences/FileTypes (via double click, no drop of a mail file on it or via a context menu of a mail file!)
• in the left-top ListView, you see known mime-types
• scroll down to the „text“ entry and click on E-mail
• now change the Preferred Application: from BeMail to NiftyApp
• click Select... and choose NiftyApp from your Apps folder (or whatever the folder is that this application happens to reside in)
• in the alert window that pops up, choose Set anyway
• close FileTypes to save your changes

Now double-click on a mail file and smile 🙂