SOLUTION: xml file error

add_filter( ‘prso_adv_uploader_reject_mimes’, ‘prso_uploader_add_mime’ );
function prso_uploader_add_mime( $allowed_mime_types ) {

$allowed_mime_types[‘xml’] = ‘application/xml’;

return $allowed_mime_types;
}

Source: https://ww.wp.xz.cn/support/topic/file-upload-error-4

https://ww.wp.xz.cn/plugins/gravity-forms-advanced-file-uploader/