Passing / in a url parameter with rawurlencode gets 404

I have a folder outside of webroot with important images. I am using $this->response->file in my controller successfully when I just pass in the file name. However I need to pass in part of the path as it is made up of id's. I know I can not pass / as it is so I use rawurlencode() which does change it to %2F. The problem now is that the controller does not get called anymore as the browser still reads it as a / and I get 

The url

http://localhost:8888/btstadmin/books/sendFile/38%2Fbackgrounds%2Forchard-left.png

Not Found

The requested URL /btstadmin/books/sendFile/38/backgrounds/orchard-left.png was not found on this server.

The view code

<?php echo (isset($book['Book']['admin_image']) && !empty($book['Book']['admin_image']) ? '<img src="books/sendFile/'.rawurlencode("38/backgrounds/orchard-left.png").'" width="100%" alt="aswq" />'  : '')  ?>

Which gives me

<img src="books/sendFile/38%2Fbackgrounds%2Forchard-left.png" width="100%" alt="aswq">

The controller which gets called and works if it is just the file name:

public function sendFile($file) {

    $file = rawurldecode($file);

    $path = ROOT.DS.'bookUploads'.DS.'Users'.DS.'ingemit/Books/'.$file;

    $this->response->file($path);

    return $this->response;

}

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