gist.php

<?php
use Symfony\Component\HttpFoundation\StreamedResponse;
use Symfony\Component\HttpFoundation\File\File;

public function streamAction()
{
  $file = new File('/path/to/largefile.ext');
  // in case you need the container
  $container = $this->container;
  $response = new StreamedResponse(function() use($container, $file) {
    $handle = fopen($file->getRealPath(), 'r');
    while (!feof($handle)) {
      $buffer = fread($handle, 1024);
      echo $buffer;
      flush();
    }
    fclose($handle);
  });
  $response->headers->set('Content-Type', $file->getMimeType());
  return $response;
}