Summary:
Based on this documentation, it shows that it's possible to add attachment by copying the link from Agent view:

To ensure that the customer sees the attachment, right click on the attachment to copy link address and paste the link into a comment surrounded by square brackets as follows: [description of attachment|attachment link]. Click preview to confirm the attachment link has copied correctly:

When users copy the link, the URL will be https://<instance-name>/secure/attachment/10200/test.pdf instead of https://<instance-name>/servicedesk/customershim/secure/attachment/10200/10200_test.pdf. Since Customer does not have access to JIRA, the attachment can't be accessed.

Steps to reproduce:

1. Customer create a new issue
2. Agent adds an attachment and right click to copy the attachment link
3. The URL will be https://<instance-name>/secure/attachment/10200/test.pdf
4. Agent adds the attachment using this format[description of attachment|attachment link]

Expected Result:

• Customer should be able to access it without having JIRA application access

Current Result:

• Customer is not able to access it

Workaround:

• Use this format [ ^description.ext ] to add the attachment

• The workaround above no longer works as even when linking the files in the chat; the file is pointing to https://<instance-name>/secure/attachment/10200/test.pdf (JIRA) and no longer point to Service Desk. For now if the customer wants to directly access the attachments; the agent have to manually provide the Service Desk URL instead; which is https://<instance-name>/servicedesk/customershim/secure/attachment/<ATTACHMENT_ID>/<ATTACHMENT_ID>_<FILE_NAME>