Process Ajax file upload with Symfony2

I used to have a ZF controller that was processing a fineuploader ajax upload. The code was simple:

$adapter = new Zend_File_Transfer_Adapter_Http();
        $filename = uniqid();
        $adapter->addFilter('Rename', APPLICATION_PATH . "/../public/temp-images/" . $filename);
        $adapter->addValidator('Size', false, array("max" => "2MB"));
        $adapter->addValidator('isImage', false);
        if ($adapter->receive()) {
            // Get mime type
            $finfo = finfo_open(FILEINFO_MIME_TYPE);
            $mimeType = finfo_file($finfo, APPLICATION_PATH . "/../public/temp-images/" . $filename);
            finfo_close($finfo);

            preg_match('/(.*)\/(.*)/', $mimeType, $matches);
            $extension = '.' . $matches[2];

Now I'm refactoring using Symfony2 and I have difficulties doing the same thing. This is what I have so far:

$form = $this->createFormBuilder()
    ->add('qqfile', 'file', array('constraints' => new File(array('maxSize' => '2M'))))
    ->getForm();


    if ($form->isValid()) {
       die('yes');
    } else {
       die('no');
    }

This is what gets sent from the browser:

------WebKitFormBoundaryYPzt2RqJ6W4awSFp Content-Disposition: form-data; name="qquuid"

b977c4b2-0edb-486b-aa86-4558275598aa ------WebKitFormBoundaryYPzt2RqJ6W4awSFp Content-Disposition: form-data; name="qqtotalfilesize"

14092 ------WebKitFormBoundaryYPzt2RqJ6W4awSFp Content-Disposition: form-data; name="qqfile"; filename="ae35e28.png" Content-Type: image/png

------WebKitFormBoundaryYPzt2RqJ6W4awSFp--

Now, I know for sure that the form will not be validated, because the POSTed data contains no name for the form. Actually, I don't even need to validate the whole form, just the uploaded file (like here Symfony2: upload a file using a file upload plugin), but how do I use validation for it?

I eventually figured it out myself.

Instead of using a form without a class, I created a class form whose getName() method returns an empty string. I set mapped=false for all the other fields except qqfile and also disabled the csrf protection for the form. In this way, the form gets properly submitted and the file input validated.