pathinfo is cool but your code can be improved:
$filename = $_FILES['video_file']['name'];
$ext = pathinfo($filename, PATHINFO_EXTENSION);
$allowed = array('jpg','png','gif');
if( ! in_array( $ext, $allowed ) ) {echo 'error';}
Of course simply checking the extension of the filename would not guarantee the file type as a valid image. You may consider using a function like getimagesize
to validate uploaded image files.