External tool generates PNG preview but it's not displayed in viewer

I'm using NConvert to generate a PNG preview of an IFX file (an IcoFX file.)

The diagnostic log shows that the PNG is generated without any problems, and the PNG is present in the temp folder.

But the PNG doesn't appear in the preview pane, which still shows the shell icon.

As far as I can tell, all the settings are correct.

The screenshot shows everything relevant that I could think of:

Opus is expecting the output file to have the name it provided; it won't know about the _1 and so won't be able to find it. You'll need to work out why the tool is adding that suffix and make it stop doing it.