I’d first suggest upgrading the Mac in the footer from macOS 10.12 to at least macOS 10.13, or to later macOS if that is supported with the particular iMac. This might address the issue here if it’s the Mac at fault, as there have been many fixes to mail over the years since 10.12. It’s unlikely macOS is involved here though, with the settings you have reported.
As for Windows questions, I’d suggest asking for assistance in a Microsoft forum. They’ll likely want to identify which mail client they’re using on Windows, whether that mail client is current or needs an update, whether Windows needs an update, whether Windows Explorer shows the file as being present and what Explorer reports about the file, and some similar basic troubleshooting steps.
Based on some very quick searches, there are many potential possibilities.
Here’s an example from Windows 11, from a Microsoft forum: https://answers.microsoft.com/en-us/windows/forum/all/photos-and-images-not-opening-on-windows-11/6d57a03d-653f-46ce-9c48-9b44735c0da9
Here are some possibilities from Windows 10: https://www.makeuseof.com/windows-10-cant-open-jpg-files/
But… This just isn’t the best spot for getting help with Microsoft Windows, which is central to his question.
Somewhat more generally, if Windows is a support issue for the person using it, and if you’re their IT support channel, maybe migration from Windows to an iPad might be worth a discussion or two?