title: File upload: How to exclude a MIME type using asserts?
body: <p>In Symfony I can accept MIME types using:</p>
  
  <pre class="lang-php prettyprint-override"><code>/**
    * @Assert\File( maxSize="10M", mimeTypes={"application/pdf", "image/png"} )
    */
  public $file;
  </code></pre>
  
  <p>But how can I exclude something from that list? Let's say, I want to allow all uploads except for PHP files?</p>