Debugging images in a WordPress upload via Ulysses

I spent hours trying to debug why Ulysses wouldn’t post my latest weeknotes to WordPress from my iPad. I kept getting an error that the document contained unsupported HEIC-format images but I couldn’t see which, as the image filenames are hidden in the Ulysses sheet. The post took ten minutes or so to upload, and each time I found I had to go to the WordPress media library and delete the images that did upload successfully so that they weren’t duplicated on my next attempt.

Eventually I found a way to view them, by dragging the images one by one from Ulysses into Gladys where the filenames became visible.

It turns out that the root cause was my having dragged-and-dropped enhanced images directly from the lightbox in Camera+ 2. Saving them to Photos first and then dragging them in from there got around the problem.