\Symfony\Component\HttpFoundation\BinaryFileResponse::prepare() sets the Content-Length header based on a simple getSize() for the file on disk. However, if the file passes through a stream filter that changes its file size, the result is that the file is delivered with the wrong header value, causing it to fail to display properly in most browsers. I discovered this while implementing an encryption layer, because it stores my files encrypted on disk and decrypts them on-the-fly for delivery. I'm able to fix the problem in BinaryFileResponse, like so:

--- a/src/Symfony/Component/HttpFoundation/BinaryFileResponse.php
+++ b/src/Symfony/Component/HttpFoundation/BinaryFileResponse.php
@@ -186,7 +186,11 @@ class BinaryFileResponse extends Response
      */
     public function prepare(Request $request)
     {
-        $this->headers->set('Content-Length', $this->file->getSize());
+        ob_start();
+        $size = readfile($this->getPathname());
+        ob_end_clean();
+
+        $this->headers->set('Content-Length', $size);

         if (!$this->headers->has('Accept-Ranges')) {
             // Only accept ranges on safe HTTP methods

The problem could probably be solved in other places, too--like \Symfony\Component\HttpFoundation\File\File::getSize()--depending on the domain model. (e.g., if File should be thought of as truly representing the file on disk, it may be a pollution of the concept to modify it to compensate for streaming behavior.)