How to filter out non-images?

Now all files will be processed as images. I hope that if only images are displayed, they will not be processed if they are not images, including the changed extension. It seems that getimagesize cannot be used here

<code>$dir = "upload/";
    if (is_dir($dir)){
        if ($dh = opendir($dir)){
        while (($file = readdir($dh))!= false){
            if(!is_dir($file)) {
              $filePath = $dir.$file;
              echo "<img src='".$filePath."'/>";
            }
        }
        closedir($dh);
        }
    }</code>