Sometimes you may need to use an external program to edit a file in the Workbench. This can occur, for example, when the Workbench has no editor for that file type.
The external program will be used as the default editor if that program is registered as the system default editor for that file type and no other editor is registered for that file type in the Workbench. For example, on most systems the default editor for JPEG files is an application for editing or viewing image files. If there is no other editor associated with .jpg or .jpeg files in the Workbench, then opening a JPEG file from the Workbench would cause the file to be opened externally, in the system default editor.
To open an external program that is not the default editor, you can use Open With > Other... from the context menu of a file. External programs can also be registered in Eclipse as the associated editor for a given file type. Use the General > Editors > File Associations preference page to register editors.