Users are not able upload attachment via public URL https://servicedesk.internal.xxx.xxx.xx , internally works fine but from outside not working.
Release : 17.X
Component : SERVICE DESK MANAGER
For external access administrator can opt for setting up DMZ server which using resides out of internal network and external users can connect to it and raise tickets. Refer below link for information. (Search for word 'DMZ')
And check the below url, it has also relevant information which can assist admin on setting up attachment on load balancer.
Disclaimer: For more assistance on DMZ or load balancer, SDM admin need to approach Services team via account manager.