The Desktop and Online version have differences when it comes to handling information, Scott.

Attachments in QuickBooks Desktop are stored in the same folder with your company file as my colleague shared. You can create a copy of those attachments since it doesn't convert to QuickBooks Online. Yes, you're correct you'll have to manually attach them again to the transactions or client information once migration is complete. Here's how to add attachments: 

Customer Profile

1. Log in to QuickBooks Online. 
2. From the Sales menu, click Customer. 
3. Locate and select the customer the file is to be attached. 
4. Click Edit at the upper-right.
5. From the Customer information window, click the Attachments tab. 
6. Click the Attachments icon and open the file to add. 
7. Hit Save. 

Add attachments to transactions

1. Click the +New button from the left menu. 
2. Select Invoice as an example. 
3. At the bottom of the Invoice page, select the Attachments icon. 

You can also look into this article for more details on how we handle attachments in QuickBooks Online.

I'll be here if you need anything else. You take care always!