problems with plupload

I have a problem with the ui widget from plupload, my javascript is:

$("#uploader").plupload({
    // General settings
    runtimes : 'html5,flash,silverlight,html4',
    url : "upload.php",

    // Maximum file size
    max_file_size : '20mb',

    // Resize images on clientside if we can
    resize : {
        width : 200,
        height : 200,
        quality : 90,
        crop: true // crop to exact dimensions
    },
    prevent_duplicates : true,
    // Specify what files to browse for
    filters : [
        {title : "Document files", extensions : "xls,xlsx,doc,docx,pdf,ppt,pptx"},
        {title : "Image files", extensions : "bmp,jpeg,jpg,gif,png"},
        {title : "Zip files", extensions : "zip,rar"}
    ],
     multipart_params : {
        "autor" : "<?=$id_user?>",
        "fecha" : "<?=$fechas?>",
        "id_recepcion" : "<?=$id_recepcion?>",
        "id_decomiso"  : "<?=$id_decomiso?>"
    },
    // Rename files by clicking on their titles
    rename: true,

    // Sort files
    sortable: true,

    // Enable ability to drag'n'drop files onto the widget (currently only HTML5 supports that)
    dragdrop: true,

    // Views to activate
    views: {
        list: true,
        thumbs: true, // Show thumbs
        active: 'thumbs'
    },

    // Flash settings
    flash_swf_url : 'js/plupload/js/Moxie.swf',

    // Silverlight settings
    silverlight_xap_url : 'js/plupload/js/Moxie.xap'
});

and upload.php is:

if (empty($_FILES) || $_FILES["file"]["error"]) {
  die('{"OK": 0}');
}

$fileName = $_FILES["file"]["name"];
move_uploaded_file($_FILES["file"]["tmp_name"], "upload/documentos/$fileName");

for some reason the documents are not saved in the destination folder, which has read / write permissions.