Symfony\Component\HttpFoundation\File\File::guessExtension PHP Method

    public function guessExtension()
    {
        $type = $this->getMimeType();
        $guesser = ExtensionGuesser::getInstance();

        return $guesser->guess($type);
    }

 public function preUpload()
 {
     if (null !== $this->photo_file) {
         // do whatever you want to generate a unique name
         $this->nom = $this->getUser()->getUsername() . '.' . $this->photo_file->guessExtension();
     }
 }