Get Attachment REST API for WebImage returns Error

I created an SR with Oracle Support but they refuse to help me, even though I'm reporting an error following their documented instructions. They said I had to come here.

We are trying to use an Oracle published REST API to get AP invoice attachments. If I manually add an attachment to an invoice in Oracle, we can successfully use "Get A File Contents" to get it. However, 99% of our AP Invoice attachments come from IDR where the images are scanned and what we see is a URL for the attachment in the UI. The "Get A File Contents" doesn't work for this. We tried "Get A FileWebImage" and get a 404 error (file not found). We tried "Get all attachments for an invoice" and this says it found