我怎么知道从流中获得的图像格式？

$image_contents = file_get_contents('path/to/image.jpg');
$image_data = imagecreatefromstring($image_contents);

$image_contents = file_get_contents('path/to/image.png');
$image_data = imagecreatefromstring($image_contents);

$image_contents = file_get_contents('path/to/image.bmp');
$image_data = imagecreatefromstring($image_contents);

$image_contents = file_get_contents('path/to/image.gif');
$image_data = imagecreatefromstring($image_contents);

$image_contents = file_get_contents('path/to/image.tiff');
$image_data = imagecreatefromstring($image_contents);