Returns the list of file types supported directly by the receiver.
An array of NSString objects. This array is empty by default. Subclasses must override to return the list of file formats they support.
The returned file types can include encoded HFS file types as well as filename extensions. When creating a subclass of NSImageRep, override this method to return a list of strings representing the supported file types. For example, the NSBitmapImageRep class implements code similar to the following for this method:
If your subclass supports the types supported by its superclass, you must explicitly get the array of types from the superclass and put them in the array returned by this method.