I appreciate you for posting your concern here in the Community, @NOWITS.

I'm here to help provide some troubleshooting steps to isolate the attaching file issue in QuickBooks Desktop (QBDT).

Most probably this issue is caused by recovering a backup file which has damaged links and created a backup. Which removes the attachments.

What you need to do is reattach the files by going to Doc Center. Here's how:

1. Browse to the location where your company file is stored.Default location: C:\Users\Public\Public Documents\Intuit\QuickBooks\Company Files
2. Make a new Attach folder.
   a. Rename Attach folder by adding "_OLD" to it.
   b. Create a new folder and name it Attach.
3. Copy the contents from the old folder.
   a. In the new Attach folder, create a folder that matches the name of the company file. Open the new folder.
   b. Create new sub-folders named Inbox and TXN.
   c. Copy and paste contents over to respective new folders from their old counterparts.
4. From the QuickBooks Company menu, select Documents > Repair Attached Documents Links.
5. Select Repair Links.

If the same thing happens, I recommend contacting our QuickBooks Desktop. They have additional tools to pull up your account and do a remote session to investigate this further. 

Here's how to contact our technical support:

1. Select QuickBooks Desktop Help from the Help menu.
2. You can also press F1 on your keyboard to bring up the same Help Panel. 
3. In order to route you to the correct support expert, we need to know what type of question you have.
4. Click Continue.
5. We’ll provide you a few options. You choose which one is best for you.

For additional reference, you can check this article to learn more about attaching files: QuickBooks Document Center: FAQs and common issues.

Please let me know how it goes by clicking the Reply button below. I'll be around to help if you have any other concerns. Have a good one.