Some PDF attachments in Confluence are replaced with a box "Unknown Attachment"

Question

For an unknown reason, as of recently some pages replaced the PDF thumbnails with a box "Unknown Attachment" though the original PDF is still present as an attachment of the page.

* What is the cause?
* How to identify such cases?
* Is there any script to fix these automatically? Otherwise, I have to replace manually.

Chihara · Answer

When I attech some pdf/xlsx to Confluence page, I see "unknown attachments" on 7.13.5. But it seems that storage format of that page are "normal" as expected.

Jiri Pik · Answer

* Confluence Server
* Insert attachment from stored attachments solves the problem

Chris Nicosia · Answer

@Jiri Pik -

* Are you using Confluence Cloud, Server, or Data Center?
* If you attempt to re-add the thumbnail view of the attachment to the page by going into edit mode, clicking on the insert images and files icon on the toolbar, and selecting the correct attachment, does the same issue occur?  (documented instructions can be found here)