public function file_error_message( $key ) {
		if ( $this->file_info( $key ) ) {
			return '';
		} elseif ( ! isset( $_FILES[ $key ] ) ) {
			return $this->__( 'File is not specified.' );
		} else {
			switch ( $_FILES[ $key ]['error'] ) {
				case UPLOAD_ERR_FORM_SIZE:
				case UPLOAD_ERR_INI_SIZE:
					return $this->__( 'Uploaded file size exceeds allowed limit.' );
					break;
				default:
					return $this->__( 'Failed to upload' );
					break;
			}
		}
	}