I am a newbie programming a Cocoa app. I read in
"You can extend the Open dialog as appropriate for your application.
For example, you could include a pop-up menu allowing users to filter
the type of files that appear in the list (see Figure 6-5). Items that
do not meet the filtering criteria would appear dimmed. The system
creates a list of native file types supported by the application to
populate the menu. You can supplement this list with custom types and
specify the default to show when the dialog opens. You should include
an All ...