void OnFormatExtensionChanged (object s, ExtensionNodeEventArgs args)
{
FileFormatNode node = (FileFormatNode) args.ExtensionNode;
if (args.Change == ExtensionChange.Add)
formatManager.RegisterFileFormat ((IFileFormat) args.ExtensionObject, node.Id, node.Name, node.CanDefault);
else
formatManager.UnregisterFileFormat ((IFileFormat) args.ExtensionObject);
}