Hi @scottrussell 

I know this is now quite a few years since you first asked this question but in case others are still looking for a solution for migrating Attachments from QuickBooks Desktop to QuickBooks Online -  I wanted to let you know that the data import and export utility fro QuickBooks called Zed Axis has been extended to provide a way to batch export attachment data from your QuickBooks Desktop company. 

This makes it possible to in migrate your attachments in batch up to QuickBooks Online.  You can check more about it in the Apps for QuickBooks Desktop marketplace

Zed