Symfony 2.7访问web目录外的图像

I read a lot of article about the problem but it seems that there has to be no answer yet to this.

So my project directory is like :
+ uploads_dir
+ symfony_proj
    - app
    - bin
    - src
    - vendor
    - web

I want to get the image inside the uploads_dir for me to use in my view page

I created twig extension that fetches the roor directory.. but it seems not to read if I put "root_dir"."../../uploads_dir".

Any suggestions ?

Here is the my twig extension part of it:

/**
     * @var container
     */
    protected $container;

    public function __construct(ContainerInterface $container){
        $this->container = $container;
    }

public function bannerFilter($filename)
    {
        $file = $this->container->getParameter('kernel.root_dir').'../../uploads_dir'.$filename;

    }

I would make the getting of a resource go through a function, this also enables your to do any kind of other checks (like is user logged in, etc).
Create a controller action that processes the request, and then in your twig you can just use a normal path() function.
Some sample code; parameters.yml

parameters:
    upload_destination: '%kernel.root_dir%/../../uploads_dir'

Sample function;

public function getFileAction($file_name)
{
    $base_path = $this->container->getParameter('upload_destination');
    $full_path = $base_path . '/' . $file_name;

    $fs = new FileSystem();
    if (!$fs->exists($full_path)) {
        throw $this->createNotFoundException();
    }

    $file_name = basename($full_path);
    $mime_type = $this->getMimeType($full_path);

    $file = readfile($full_path);
    $headers = array(
        'Content-Type'     => $mime_type,
        'Content-Disposition' => 'inline; filename="'.$file_name.'"');
    return new Response($file, 200, $headers);
}

protected function getMimeType($file)
{
    if ('jpg' === substr($file, -3)) {
        $best_guess = 'jpeg';
    } else {
        $guesser = MimeTypeGuesser::getInstance();
        $best_guess = $guesser->guess($file);
    }

    return $best_guess;

}

In your twig;

<img src="{{ path('whatever_you_called_your_route', {'file_name': 'my_file.jpg'}) }}" />

I got passed this by using /uploads_dir/. I hopes this helps.