Hello there, sequoiafinefinish. I appreciate you for joining here. 

If you have tried following the recommendation above, you can follow the alternative way of uploading attachments to transactions in QuickBooks. 

Take a look at this step-by-step visual below to see how to go about adding attachments:

1. Go to the Gear icon.
2. Select Lists under the Lists column.
3. Click the clip icon or Attachment. Select the file you want to upload in the program.

Once done, put a checkmark on the attachment you've uploaded. Then, select the Batch actions drop-down arrow and choose the Create invoice or Create expense, depending on your need. Here's the sample screenshot:

If the same thing happens, I'd still recommend contacting our support team to further investigate this matter. You can follow the steps shared by my colleague ReymondO above.

I'm adding this article for more information and tips when doing the process: Attachments in QuickBooks Online.

You can update me here on how these steps go. Also, you can post other concerns about QuickBooks. I'll help you in any way that I can.