If you are using version 5.8.16 or 5.8.17, or have used those versions in the past without running a script to fix the affected attachments, then you are likely affected by CONF-39975. Please see that ticket instead.

In some cases, it is legitimate for the message "Unknown attachment" to appear. For example:

• If you upload an attachment to a page and then delete it from the attachments list without removing the macro which references it, the macro will show "Unknown attachment".
• If you upload an attachment on Page-A and then reference it in a macro on Page-B, then delete the original Page-A, then the attachment will also be deleted. This causes the Page-B macro to display "Unknown attachment"

If you observed "unknown attachment" after one of these scenarios, this is not a bug, but rather expected behaviour.

If you observed "unknown attachment" by any other scenario, it is likely a bug. If you find a way to reproduce this issue, please let us know by commenting below, with details of how to reproduce the issue and your version of confluence. If you observe something different than an "unknown attachment" message but attachments do fail to download, please let us know exactly what you see. If you observe this issue but are not on and have never been on Confluence 5.8.16 or 5.8.17, please let us know below.

If you observe this issue you may try resolving the broken attachments using the same instructions outlined in CONF-39975 - let us know if it works or not for you.