This problem is been caused by the feature JSD-43 that's missing from JIRA Service Desk. Though, you can work around it by specifically referring to the attachment in a comment, i.e.
To compound on that though, there appears to be a bug when displaying images using the above wiki markup, you can workaround that problem by using:
Please feel free to add your comments on that feature and vote for it to make it more noticeable to our developers. Also refer to Implementation of New Feature Policy for more information on Atlassian's approach to the development of these improvements.
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