iPhone 6 and iPhone X: Mail app not showing attachments correctly
As the Internet search has yielded that the problem is also being faced by others, but hasn't yielded any solution, it's most likely a bug with the stock Mail app.

A likely resolution would be to use the official email client app from the email service provider (Outlook in this case). Official app is more likely to work bug free compared to Apple's Mail app.

The stock Mail app can be easily uninstalled on devices running iOS 10 and later as outline in this Apple Support document:


Remove built-in Apple apps from the Home screen on your iOS 10 device or Apple Watch.