can't upload file using Uploadify and codeigniter

I am using codeigniter and uploadify. I am not using the built in uploadify.php, instead I am using my own function in my controller which I feel more suit with my application workflow. I have install uploadify and test it with it's built in .php file. it works good. But when I tried to use my own script, It couldn't upload the file. The error said :

A PHP Error was encountered

Severity: Warning

Message: Illegal offset type in isset or empty

Filename: libraries/Upload.php

Line Number: 147

You did not select a file to upload.

what is it mean? I am sure I've selected the file... for the clarity of my question, here's my code : The View:

 <?php echo form_open_multipart('profile/do_upload_song');?>
 <input id="uploadifyit" type="file" name="filedata" />
 <a href="javascript:$('#uploadifyit').uploadifyUpload();">Upload Files</a>
 <?php echo form_close(); ?>

The .JS:

$("#uploadifyit").uploadify({
        'uploader'       : 'js/jquery/jquery-plugins/uploadify/uploadify.swf',
        'script'         : 'profile/do_upload_song',
        'cancelImg'      : 'js/jquery/jquery-plugins/uploadify/cancel.png',
        'folder'         : 'media',
        'scriptAccess'  : 'always',
        'displayData'   : 'speed',
        'auto'           : false,
        'multi'          : true,
        'fileDataName'  : 'Filedata',
        'fileExt'     : '*.mp3',
        'method'    : 'post',
        'removeCompleted' : false,
        'onComplete': function(event, queueID, fileObj, reposnse, data) {

            $('#fileinfotarget').append('<p>'+reposnse+'</p>');
        }
        });

The Controller:

function do_upload_song()
    {

        $config['upload_path'] = './media/';
        $config['allowed_types'] = 'mp3';
        $config['max_size']    = '144400';
        $config['max_width']  = '1024';
        $config['max_height']  = '768';

        $this->load->library('upload', $config);

        if ( ! $this->upload->do_upload($_FILES['Filedata']))
        {
            echo $this->upload->display_errors();

        }    
        else
        {
            $data = array('upload_data' => $this->upload->data());

            print_r($data);
        }


    }

What's wrong with my code? Any advice guys? Thanks