TYPO3 Core

In case of using frontend upload in an extbase, I use the storage for saving the uploaded files.
While trying to create the needed folder for using $storage->addFile() the properyMapper throws exeption

#1297759968: Exception while property mapping at property path "":Property "identifier" had no setter or constructor argument in target object of type "TYPO3\CMS\Core\Resource\Folder" 

Following code does this error:

$folderType = array(
     'storage' => $storage,
    'identifier' => $mountpoint->getPath(),
    'name' => $mountpoint->getTitle()
);
/** @var \TYPO3\CMS\Core\Resource\Folder $folder */
$folder = $this->propertyMapper->convert($folderType,'TYPO3\\CMS\\Core\\Resource\\Folder');

Creating the object with "new" and assigning the parameters to the constructor does the thing correct and uploads the files to the specified folder.
My current Typo3 is 7.5