Very similar to WEB-6467, just the way of doing it is different.
Clicking to display attached files when creating a ticket makes the button disappear.
Steps
Select a bug report without attachements
Create a new bug report
Attach a file
Click on the "Attachement" button
The button disappear
Thanks for the report! This is Jira behaviour, and if it's considered an issue, it would be tracked on the Jira page for Jira. Mojang does not manage the bug tracker base design/functionality itself.
Closing as invalid.