step_2_manage_upload example

'<p>' . __( 'The active theme does not support uploading a custom header image.' ) . '</p>',
                403
            );
        }

        if ( empty( $_POST ) && isset( $_GET['file'] ) ) {
            $attachment_id = absint( $_GET['file'] );
            $file          = get_attached_file( $attachment_id, true );
            $url           = wp_get_attachment_image_src( $attachment_id, 'full' );
            $url           = $url[0];
        } elseif ( isset( $_POST ) ) {
            $data          = $this->step_2_manage_upload();
            $attachment_id = $data['attachment_id'];
            $file          = $data['file'];
            $url           = $data['url'];
        }

        if ( file_exists( $file ) ) {
            list( $width, $height, $type, $attr ) = wp_getimagesize( $file );
        } else {
            $data   = wp_get_attachment_metadata( $attachment_id );
            $height = isset( $data['height'] ) ? (int) $data['height'] : 0;
            $width  = isset( $data['width'] ) ? (int) $data['width'] : 0;