Do you still have the Jira Server instance available to you? If so I would recommend returning to those issues on the Jira Server instance to make sure that the attachments exist and are functional there as well.
The Server to Cloud migration process should have included these as flat files to be imported. Jira stores attachments in the $JIRAHOME/data/attachments/ folder. Part of the process of migrating to Cloud requires you to zip the contents of that folder and include this in the import process. I don't yet have an explanation as to why the attachments from 2010 or before would not exist here. But it would help to check that the server had these at the time of import first. If it does, then it would also be a good idea to try to check and make sure that your backup zip file contained these as well.
If both are true, then we would likely want to dig into this problem deeper by opening a support request to investigate the details here. It might be possible there are logs of the import still if this was a recent event. If you like I can do this on your behalf, or you can create a support request directly at https://getsupport.atlassian.com
If you still have the Jira Server available, a quick way to find these would be to use the Jira search JQL of:
attachments is not empty and created <= '2010/12/31'
This should be able to show you issues in Jira created on or before that date that also have at least 1 attachment.
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