In GetSimpleCMS 3.3.15, admin/upload.php blocks .html uploads but Internet Explorer render HTML elements in a .eml file, because of admin/upload-uploadify.php, and validate_safe_file in admin/inc/security_functions.php.
Add your gear to cvedb and we'll alert you only when get-simple ships something exploited.
Check my exposure →This product uses data from the NVD API but is not endorsed or certified by the NVD. Informational only; not professional security advice.