There is an issue with Jira Service Desk that attachments sent to a customer in a ticket comment require the customer to log in to the Help
...
Centre to view it.
There is a feature request to allow attachments to be sent directly in the email instead of link to the portal, here:
In the meantime, customers can use their email address to log in, and use the "Forgot your password?" link to create a password and get into the portal to view attachments.