To associate editors with various file types in the Workbench:
General > Editors > File Associations preference page.When you associate an internal editor with a file type, that editor opens in the editor area of the Workbench. For example, if you double-click a file in the Project Explorer or an entry in the Bookmarks or Tasks view it opens in the editor area.
Editors that support OLE document mode can also run in the editor area of the Workbench.
Tip: You can choose to override your default editor selections by selecting Open With from the pop-up menu for any resource in one of the navigation views.

Opening files for editing
Editing files outside the Workbench
Linking the Project Explorer view to the active editor