Images renamed to image.png.swift-original

Hello there.

A customer recently run into a problem where the images of his website were all renamed to “image.png.swift-original”. I suspected it might be one of his image optimizing plugins, so we tried to restore the page from backup, disable all the plugins and see what happens. It didn’t help. Seems like we have no choice but to manually fix the image names ??
The WordPress site is installed on a shared cPanel host, and no other customer complained about a similar issue, so I would rule out server-side problems for now.
Google yields no results, only a couple fishy-looking websites suffering from the same issue are indexed. I’m confused. Has anyone experienced anything similar?

The page I need help with: [log in to see the link]