FTP Attachments® Data flow 

User uploads an attachment using FTP Attachments®, initially attachment 
uploads from user's machine to Application Server and followed on from 
Application Server it ends up in Storage Server.

Any user when attempts to view an attachment using FTP Attachments® 

which is stored on Storage Server, data flow would be attachment will get 

downloaded to Application Server then  from Storage Server it moves to

Application Server and downloads their and at the end to user’s machine.

In the above described manner end user won't get any direct access to 

Storage Server at any point of time; which is an added layer of security.

Additionally, Salesforce users can only view such attachments to which they

have access rights as per security configuration for FTP Attachments® object.

                                               Figure: 3.1