unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, Registry, StdCtrls;

type
  TForm1 = class(TForm)
    Memo1: TMemo;
    procedure FormCreate(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.FormCreate(Sender: TObject);
var
    Reg: TRegistry;
    FileExt:String;
    FileType:String;
    MIMEType:String;
    ExecName:String;
begin
    FileExt:='.test';
    FileType:='Test File';
    MIMETYPE:='text/plain';
    ExecName:=Application.ExeName +' %1';

    if ParamCount>0 then
    begin
        self.Memo1.Lines.LoadFromFile(ParamStr(1));
    end
    else
    begin
        Reg := TRegistry.Create;
        Reg.RootKey :=HKEY_CLASSES_ROOT;
        Reg.OpenKey(FileExt, True);
        Reg.WriteString('', FileType);
        Reg.WriteString('Content Type', MIMEType);
        Reg.OpenKey('shell\open\command',True);
        Reg.WriteString('', ExecName);
        Reg.CloseKey;
        Reg.Free;
    end;
end;

end.