Clicking on attachments link from JSD notification requires login

Summary

Attachments that are embedded into comments, when sent as a notification to a JIRA Service Desk customer, are not accessible without login. Considering that most customers in JSD are created automatically because of public sign up, these customers don't really have the password to login to view these attachments directly. This problem is similar to https://jira.atlassian.com/browse/JSD-1361 and is possibly a regression.

Steps to Reproduce

1. Create a JIRA Service Desk ticket.
2. Attach an attachment to the ticket.
3. Change the reporter to JIRA Service Desk customer that uses one of your emails.
4. Embed said attachments in a comment via [^test.png] so that it triggers a notification to the reporter.
5. When you receive the notification, copy the attachment link and load it on *private/incognito browsing'.

Expected Result

Customer should be able to view the attachments without login.

Current Result

Customer have to login first to view the attachments.