var

  Bytes: TBytes;

  Stream: TBytesStream;

begin

  with System.NetEncoding.TBase64Encoding.Create do

  try

    Bytes := Decode(System.IOUtils.TFile.ReadAllBytes('video.txt'));

  finally

    Free;

  end;

  Stream := TBytesStream.Create(Bytes);

  try

    Stream.SaveToFile('video.mp4');

  finally

    FreeAndNil(Stream);

  end;

end;