[TYPO3-mvc] Validation Error File Upload

[Sören Kracker]

Hello,
I always get this error when submitting a form with file upload in backend:

An error occurred while trying to call 
Tx_Placeholder_Controller_CatalogController->createAction()
1245107351: Validation errors for argument "newCatalog"
newCatalog: Validation errors for property "filename"

But I haven't set any validation for the filename except NotEmpty but I 
select a file, so it shouldn't be empty. Does anybody have an idea 
what's going wrong? Maybe debugging hints too? I'm new to extbase/fluid.

Controller:
/**
      * action create
      *
      * @param  $newCatalog
      * @return string The rendered create action
      */
     public function createAction(Tx_Placeholder_Domain_Model_Catalog 
$newCatalog) {
         ...
     }

TCA:
'filename' => array(
             'exclude' => 0,
             'label' => 'filename',
             'config' => array(
                 'type' => 'group',
                 'internal_type' => 'file',
                 'uploadfolder' => 'uploads/tx_placeholder',
                 'allowed' => '*',
                 'disallowed' => 'php',
                 'size' => 1,
             ),
         ),

Model:
/**
      * filename
      *
      * @var string
      * @validate NotEmpty
      */
     protected $filename;

/**
      * Returns the filename
      *
      * @return string $filename
      */
     public function getFilename() {
         return $this->filename;
     }

     /**
      * Sets the filename
      *
      * @param string $filename
      * @return void
      */
     public function setFilename($filename) {
         $this->filename = $filename;
     }