After the upgrade, we cannot open the attached files when we try to open them.
If we try to open the files which we created after the upgrade, we can open the attached files successfully.
What is the root cause and solution for the issue ?
This is due to the fact that you deleted the custom object which you define the attachment attribute.
If you delete the custom object, then the attributes and instances for the custom object are deleted.
Please do not delete the custom object before the upgrade if you avoid this.