I've done this (set the renderer to wiki style), but still facing similar issue. However my problem seems to apply to only images files (I have tried with png and jpg) as e.g. text file attachments are shown properly. The issue is the same whether customer adds the file or service desk adds the file.
With images the text is displayed: !image.png!
But with e.g. Word documents, there is the link to file which is downloadable also for customer.
Service Desk Agent can access and see the files correctly. Thus, the problem is only on customer portal side.
I am having JIRA v8.7.1 and JIRA Service Desk 4.7.1