Microsoft Excel files can have a Content Type of application/vnd.ms-excel which causes an exception to be raised in the Zend\Mail\Header\ContentType file [line 105];

Zend\Mail\Header\ContentType::setType expects a value in the format "type/subtype"; received "application/vnd.ms-excel"

if (!preg_match('/^[a-z_-]+\/[a-z_-]+$/i', $type)) {