为什么$ file在自定义symfony文件验证器中为空？

I am setting a custom file validator for a file input field in Symfony 1.4 (Doctrine)

$this->setValidator('filename', new sfValidatorFile(array(

'mime_types' => 'web_images',
'path' => sfConfig::get('sf_upload_dir').'/animals',
'validated_file_class' => 'CustomValidatedFile',
'required' =>false
))); 

I then want to override the save method to generate some thumbnails:

class CustomValidatedFile extends sfValidatedFile {

private $savedFilename;

// Override sfValidatedFile's save method
  public function save($file = null, $fileMode = 0666, $create = true, $dirMode = 0777)                 {

$this->savedFilename = $this->generateFilename();
// This makes sure we use only one savedFilename (it will be the first)
if ($this->savedFilename === null ) {

$this->savedFilename = $file;
}

$thumbnail = new sfThumbnail(150,150);
$thumbnail->loadFile($this->getTempName());
$thumbnail->save(sfConfig::get('sf_upload_dir').'/thumbnails/thumb_'.$this->savedFilename);
// Let the original save method do its magic :)

return parent::save($this->savedFilename, $fileMode, $create, $dirMode);
  }
  }

This fails with the error "The file "" is not readable." This is coming from sfThumbnailPlugin because the value of $file is null.

My question is why is $file null?

I think its because $file doesn't exist yet - only a temp file exists at this stage which can be accessed via $this->getTempName()