I could not find any documentation on how to make attachments "advanced available" same as the app servers, so I went ahead and set it up myself and thought I would share how I got it to work. We used F5 Load Balancer, and below is the setup I did in Service Desk Manager.
Background Server name: BKG
Standby Server Name: SBY1
Application Server Name: APP1
Application Server Name: APP2
Application Server Name: APP3
F5 Load Balancer Name: f5loadbalancer.name
DFS Server to host the files for attachments on a UNC path available to the App servers.
Login to Background Server Web GUI for CA Service Desk Manager
Administration Tab -> System -> Servers
Host Name: f5loadbalancer.name
Server Type: Application Server
Attachment Servlet Path: http://f5loadbalancer.name:8080/CAisd/UploadServlet
In our instance we actually used https://f5loadbalancer.name/CAisd/UploadServlet and redirected the traffic from external / internal to the correct place ie port 8080 etc. on the F5 side
Administration Tab -> Attachments Library -> Repositories
Edit Repository that will use the UNC Path / AA Availability server configuration ie. Service Desk / Images etc..
Click on UNC Credentials
Symbol: Provide a friendly name here
Password: Domain Password goes here
Background Services: Daemon Only
This is where the important part is:
Servlet Server: f5loadbalancer.name
Click Save - repeat for all Attachment Repositories needed
The above are the important items - the rest can be configured as needed
This allows the attachments to be available in the same manner as the application servers. We also setup cookie stickiness across the servers so that it goes to the same server for attachments / exports / CA SDM GUI.
Happy AA :-)