Symptoms

Some images will not display correctly on a Confluence page, and will display "Unknown Attachment" instead of the image. This behaviour may be intermittent between users, or anonymous users.

Diagnosis

You can examine the Confluence Storage Format of a given page that exhibits this problem. In particular, you'll be looking for an <ac:image> macro, like this:

<ac:image>
	<ri:attachment ri:filename="MyFileName.png">
		<ri:page ri:content-title="My Page" ri:space-key="TST"/>
	</ri:attachment>
</ac:image>

The ri:page element tells Confluence to look for the attachment on another page in the system, rather than looking at the current page for the attachment.

If the source page (in this example, "My Page" in the "TST" space) is restricted or unreachable due to permissions, it may cause problems for some viewers of this page. There may also be another problem at play which we haven't been able to identify.

There's a few ways you can potentially identify the cause:

1. Does the image display inconsistently for all users of Confluence? If it's not consistent, make note of the group memberships for people whom it does work for, versus the people it does work for.
2. If the page is in an anonymously accessible space, Incognito Mode (or Private Browsing) can help you determine if the problem is permissions related
3. If the image is located on another page, check the space permissions, as well as any restrictions on that page (or the parent pages), as they will prohibit access to attachments as well.

If the following stacktrace appear in the logs, please go to  workaround 2:

2015-03-18 10:30:02,623 ERROR [http-bio-443-exec-392] [atlassian.event.internal.AsynchronousAbleEventDispatcher] run There was an exception thrown trying to dispatch event [com.atlassian.confluence.event.events.content.attachment.AttachmentViewEvent[source=com.atlassian.confluence.servlet.download.AttachmentDownload@977bc3a]] from the invoker [SingleParameterMethodListenerInvoker{method=public void net.customware.confluence.plugin.tracking.TrackingEventListener.onEvent(com.atlassian.confluence.event.events.content.attachment.AttachmentViewEvent), listener=net.customware.confluence.plugin.tracking.TrackingEventListener@444478a1}]

Cause

This can be caused by copy and pasting the image from one Confluence page to another. While this is the most common cause, we've had reports from customers where this wasn't happening. If you're running into these problems, or can reliably reproduce these problems, please contact Atlassian Support - we'd love to hear from you!

Workaround 1

Delete the image from the remote page, and re-attach it to the page, and insert the image again.

If the remote page was deleted, restore it from the Space Trash.

Workaround 2

Disable the tracking plugin from ServiceRocket.