Ajaxsubmitbutton File Field Undefined Index

I want to send a file to my ajax Controller, but it sais "Undefined index: download_file"
I googled around for 2 hours but i cant understand the problem behind.

My View:

<? $form = $this->beginWidget('CActiveForm', array('id' => 'input-form-adddownload', 'htmlOptions' => array('enctype' => 'multipart/form-data'))); ?>
<input id="fileUploader" type="file" name="download_file" value="">
            <?=
            CHtml::ajaxSubmitButton(
                    'Do it', array('ajax/SaveNewDownload'), array(
                'type' => 'post',
                'beforeSend' => 'function(){$("#error_download").html("loading...");}',
                'success' => 'js:function(msg){$("#error_download").html(msg);}',
                    ), array('class' => 'ans_submit_button', 'id' => 'btn_new')
            );
            ?>
        <td><span id="error_download"></span></td>

<?php $this->endWidget(); ?>

Ajax Controller:

    public function actionSaveNewDownload() {
        print_r($_FILES["download_file"]); 
           echo "test string";
    }

when i change the file field to text and test something to output then everything works great.
i want to store a file in my ajax method.

well i could build a big workaround with own javascript code and everything but i dont understand the main problem here, why this seems not to be possible with the ajaxsubmitbutton?

thx