Problem With Cuploadedfile And Android Http Request

Hi,
i'm trying to upload a file named uploaded_file with an android http request.

My environment is:

MAMP 2.1.1 with PHP 5.4.4 and Mountain Lion 10.8.3, MySQL database UTF8

If i use move_uploaded_file, everything works fine, but i want to use CUploadedFile.

My model Media is

class Media extends CActiveRecord
{
	public $uploaded_file;
        .....

	public function rules()
	{
		// NOTE: you should only define rules for those attributes that
		// will receive user inputs.
		return array(
			array('Name, Title', 'length', 'max'=>500),
			array('CreatedAt, LastEdit, Filename', 'safe'),
			// The following rule is used by search().
			// Please remove those attributes that should not be searched.
			array('ID, CreatedAt, LastEdit, Name, Title, Filename', 'safe', 'on'=>'search'),
			array('uploaded_file', 'file', 'types'=>'jpg,jpeg,gif,png'),
		);
	}

        ...
}

and in the controller i have

public function actionUploadImage(){
...
$media = new Media;
$media->uploaded_file=CUploadedFile::getInstance($media,'uploaded_file');
...
}

but nothing is uploaded.
if i try

		error_log($media->uploaded_file);
		error_log($_FILES['uploaded_file']['name']);

i see the second one but the first is empty.

I've also tried to upload the file using a form

$form = $this->beginWidget(
'CActiveForm',
array(
'id' => 'upload-form',
'enableAjaxValidation' => false,
'htmlOptions' => array('enctype' => 'multipart/form-data'),
)
);
// ...
echo $form->labelEx($model, 'uno');
echo $form->fileField($model, 'uploaded_file');
echo $form->error($model, 'tre');
echo CHtml::submitButton('Carica');
$this->endWidget();

And it works.

Where am i wrong?

Thank you very much.

Giacomo