An OnDemand user has received a backup of their instance's attachment files but they are unable to extract them due to the error: "Invalid archive header found in attachments.tar".
When this error occurs with a specific app, it is recommended to try a couple of different tools to extract the file such as 7-Zip, WinRAR, WinZip. Additionally, note that the backup folder is usually nested so one would need to extract the folders inside the tar file as well.
If the issue persists even when using different tools, or if one notices the backup is missing files, it might be an indication that the attachments archive could be corrupted. Thus, one would need to Contact Support explaining the situation and to request a new backup following the process in Requesting a Database Backup of Your Kayako Classic OnDemand Instance.