Attachments can not be directly dragged and dropped from an email to Risk Cloud
You currently can not drag and drop an attachment from an email directly into Risk Cloud or any web based applications without custom code or a 3rd party plugin. Some email clients do not use a standard drag and drop format which means that other web applications like Risk Cloud will not be able to recognize the drag and drop.
The best way to add attachments in Risk Cloud is to first download it to your local drive and then drag and drop into Risk Cloud from your local drive.