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function InstallExt(Extension, ExtDescription, FileDescription,
OpenWith, ParamString: string; IconIndex: Integer = 0): Boolean;
var
  Reg: TRegistry;
begin
  Result := False;
  if Extension <> '' then
  begin
    if Extension[1] <> '.' then
      Extension := '.' + Extension;

    Reg := TRegistry.Create;
    try
      Reg.RootKey := HKEY_CLASSES_ROOT;
      if Reg.OpenKey(Extension, True) then
      begin
       Reg.WriteString('', ExtDescription);
       if Reg.OpenKey('\' + ExtDescription, True) then
       begin
         Reg.WriteString('', FileDescription);
         if Reg.OpenKey('DefaultIcon', True) then
         begin
           Reg.WriteString('', Format('%s,%d', [OpenWith, IconIndex]));
           if Reg.OpenKey('\' + ExtDescription + '\Shell\Open\Command', True) then
           begin
             Reg.WriteString('', Format('"%s" "%s"', [OpenWith, ParamString]));
             Result:=True;
           end;
         end;
       end;
     end;
  finally
    Reg.Free;
  end;
  end;
end;					

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Reg.WriteString('', Format('%s,%d', [OpenWith, IconIndex]));					

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If InstallExt('*.butler', 'DeckButler-Datei', 'DeckButler', ParamStr(0), '"%1"', 0)
Then ShowMessage('Programm wurde verknüpft.');					

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procedure TForm1.FormCreate(Sender: TObject);
begin
 If (ParamCount>0) and
    (FileExists(ParamStr(1))) then
   Memo1.Lines.LoadFromFile(ParamStr(1));
end;