register filetype with application

found that code below here at EE, but

a) using WIN 7 and application has no admin rights this code makes an AV !

b) Application with ADIM Rights, no AV but also no effect

c) how to unregister a Filetype and a extension  

{**************************************************************
 *
 *  source : http://www.experts-exchange.com/Programming/Editors_IDEs/Delphi/Q_24973381.html
 *
 **************************************************************}

procedure RegisterFileType(ExtName:String; AppName:String) ;
var
   reg:TRegistry;
begin
  reg := TRegistry.Create;
  try
   reg.RootKey:=HKEY_CLASSES_ROOT;
   reg.OpenKey('.' + ExtName, True) ;
   reg.WriteString('', ExtName + 'file') ;
   reg.CloseKey;
   reg.CreateKey(ExtName + 'file') ;
   reg.OpenKey(ExtName + 'file\DefaultIcon', True) ;
   reg.WriteString('', AppName + ',0') ;
   reg.CloseKey;
   reg.OpenKey(ExtName + 'file\shell\open\command', True) ;
   reg.WriteString('',AppName+' "%1"') ;
   reg.CloseKey;
  finally
   reg.Free;
  end;

  SHChangeNotify(SHCNE_ASSOCCHANGED, SHCNF_IDLIST, nil, nil) ;
end;

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