Problem

If the attachment folder is accidentally deleted and later restored, the thumbnail image for most of the attachments cannot be viewed in the old tickets. Newly added attachments won't have this issue.

Solution

Run the below query in the DB to update the thumbnailable value to 1 for the required file types that you want to see thumbnail for:

UPDATE fileattachment set thumbnailable = 1 where thumbnailable = 0 and mimetype in ('text/html','image/png')

The below query can be used to find the distinct types of attachments available in the Jira

select distinct mimetype from fileattachment ;