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);
        reg.CloseKey;
        if reg.OpenKey('\' + ExtDescription, true) then
        begin
          reg.WriteString('', FileDescription);
          reg.CloseKey;
          if reg.OpenKey('DefaultIcon', true) then
          begin
            reg.WriteString('', Format('%s,%d', [OpenWith, IconIndex]));
            reg.CloseKey;
            if reg.OpenKey('\' + ExtDescription + '\Shell\Open\Command', true) then
            begin
              reg.WriteString('', Format('"%s" "%s"', [OpenWith, ParamString]));
              result := true;
              reg.CloseKey;
            end;
          end;
        end;
      end;
    finally
      reg.Free;
    end;
  end;
end;