overriding UIManagedDocument -savingFileType

Question

I'm trying to override UIManagedDocument's -savingFileType so that my app's files can be of a type other than a folder.

-(NSString*)savingFileType{
    return @"com.myCo.myApp";
}

- (NSString *)fileNameExtensionForType:(NSString *)typeName saveOperation:    (UIDocumentSaveOperation)saveOperation{
    return @"myDocType";
}

This causing a crash, but as far as I can tell from the documentation of UIDocument, this should be enough to do the trick. Any ideas?