It is setup that when you create a incident you can attach multiple files, however, when a user adds a comment, and attach a file there, they can only select one file. I notice questions being asked aorund 2018-2019 with the same issue, saying it is not a function that is support. However, I noticed this statement in release notes in 2020
Dynamics 365 Monthly Update-July 2020
- Portal Timeline control doesn't honor "Allow multiple files" option on Entity form and only allows a single file to be uploaded at a time. After this change, Portal Timeline control will honor the setting and will enable multiple files to be uploaded at once.
So I ma not sure why we are unable to add more than one file when added a portal comment via the portal.