Hi mc1870.
Attaching files to your transactions is a great way to keep information you may need associated with it. I can help go over this with you.
QuickBooks Online makes keeping track of information easy by having the option to attach a file to your transactions. This allows you to quickly pull it up when you open the invoice or go to the Document Center. When you update your company file to a new year or move its location breaks the link between the attachment and the invoice. Great job already trying to repair the link but if the company file was moved to a new computer or the location where the attachments were saved is moved you'd need to manually attach them again. This is due to the attachments not being saved to the company file but to a folder where your company file is located. I recommend setting up your company file on one computer and setting up the multi user mode so everyone can access it from their computers using a network. In order to set up multi user mode you can follow the steps in this article.
If you have any questions feel free to reach back out and we'd be happy to help!