Cakephp图像 - 不能确定mimetype

cakephp 2.3

我上传了一张图片,但我说错了:

Can not determine the mimetype.

Error: An Internal Error Has Occurred.

在我的模型上,这是我的$ validation的一部分

'file_name' => array(
            'uploadError' => array(
                'rule' =>'uploadError',
                'message' => 'Your image upload failed',
                'allowEmpty' => FALSE,
                //'required' => false,
                //'last' => false, // Stop validation after this rule
                //'on' => 'create', // Limit validation to 'create' or 'update' operations
            ),
            'mimeType' => array(
                'rule' => array('mimeType', array('image/gif', 'image/png', 'image/jpg', 'image/jpeg')),
                'message' => 'Please only upload images (gif, png, jpg).',
                'allowEmpty' => FALSE,
                ),
            'fileSize' => array(
                'rule' => array('fileSize', '<=', '2MB'),
                'message' => 'Your image must be less than 2MB or(2048ko).',
                'allowEmpty' => FALSE,
                ),
            'processCoverUpload' => array(
                'rule' => 'processCoverUpload',
                'message' => 'Unable to process cover image upload.',
                'allowEmpty' => FALSE,
                ),
            'unique' => array(
                'rule' => 'isUnique',
                'message' => 'This file name is already exist in your folder',
                'required' => 'create',
                'allowEmpty' => FALSE,
                ),
        ),

我只允许3种类型的mimetype.任何帮助？