The option "Attach files" to the comment disappears once you have commented the ticket once. If you want to attach a file to another comment you have to refresh the page.
The steps would be:
1. Open a ticket
2. Post a comment
3. The next comment will not have the option to attach a file
I'm not sure if this is intentional but it's really helpful to attach different files to different comments, as some of them might be private.