PLEASE READ FIRST:
This is NOT for attachment previews that do not display in comments - that ticket is:
https://jira.atlassian.com/browse/CONFCLOUD-76315
This is also NOT for attachment previews that do not display in the new fabric editor - that ticket is: 
https://jira.atlassian.com/browse/CONFCLOUD-76351

Update:

Issue has been resolved for new migrations. 

For migrations that have already taken place, we've isolated the issue and are working on a fix.  The ticket has been reopened and will remain open until the fix is merged.  

In the meantime, for pages that are immediately vital - deleting and re-uploading attachments is the only workable triage fix at the moment for customers that cannot re-migrate.  

Issue Summary

After migration from server to cloud, attachment thumbnails no longer show previews of their content.  

Steps to Reproduce

1. Migrate from server
2. Try to view thumbnail of attachment

Expected Results

Attachment thumbnail is visible.

Actual Results

Attachment thumbnail shows as 'couldn't generate preview'

Workarounds

1. Delete and re-upload attachments