//AValue 为图片文件流头两个字节
function GetImageExt(AValue: Cardinal): String;
begin
  case AValue of
    $4D42, $424D: Result := '.bmp';
    $D8FF, $FFD8: Result := '.jpg';
    $4947, $4749: Result := '.gif';
    $050A, $0A05: Result := '.pcx';
    $5089, $8950: Result := '.png';
    $4238, $3842: Result := '.psd';
    $A659, $59A6: Result := '.ras';
    $DA01, $01DA: Result := '.sgi';
    $4949: Result := '.tiff';
    else Result := '';
  end;
end;