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You'll have to create a copy of those attachments to transfer them from QuickBooks Desktop (QBDT) to QuickBooks Online (QBO). Then manually attach them again to the transactions or your client’s information in QBO. 

The “Attach” folder can be found in the same location as your QuickBooks Company file. For more details about this process, you can refer to this article: QuickBooks Document Center: FAQs and common issues.

Then to add attachments in your QBO account:

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

To 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.

Additionally, I got you this article that you can check to learn more about the limitation on exporting QBDT to QBO: What to expect when you switch from QuickBooks Desktop to QuickBooks Online.

Please tag me in if you have other concerns or questions about migrating from QBDT to QBO. I’ll be more than happy to help you out. Have a great and wonderful day ahead!