Creates and returns a document object of a given document type from the contents of a file.
Creates and returns an NSDocument object for document type docType from the contents of the file fileName, which must be a fully qualified path. The returned object is not retained. Returns nil if the NSDocument subclass for docType couldn’t be determined or if the object couldn’t be created. This method calls the NSDocument method initWithContentsOfFile:ofType: and is called by openDocumentWithContentsOfFile:display:.