I can see the importance of being able to migrate attachments in bulk. I'll elaborte some details on this, rabidchkn.

QuickBooks Desktop is a more traditional accounting software that you download and install. While QuickBooks Online is best for businesses that need real-time collaboration and access to their QuickBooks files from any device. But both software caters to the same type of businesses. 

For now, moving attachments in bulk is unavailable in QuickBooks. While this isn't a possible option, you can search and use a third-party application. You can also search online if it's not available in our QuickBooks Apps Center. This way, you'll be able to select an app that suits your business needs and helps you in moving your files (attachments) from desktop to online. 

Moreover, you may notice differences in how your data appears once you've moved them to QBO. To learn more, check out this article for more details: Learn How Features And Data Move From QuickBooks Desktop To QuickBooks Online.

Let me know if you have any other concerns when migrating your data from desktop to online. The Community is always here to help you 24/7. Have a great day!