Since latest Mojave update - I cannot make attachments to Hotmail or Gmail emails

Is this about using in Mail app or safari ?

Have you tried safe mode to troubleshoot?

How to use safe mode on your Mac - Apple Support (Takes longer than normal boot)

Note: Safe mode prevents your Mac from loading certain software as it starts up, including login items, system extensions not required by macOS, and fonts not installed by macOS. It also does a basic check of your startup disk, similar to using First Aid in Disk Utility. And it deletes some system caches, including font caches and the kernel cache, which are automatically created again as needed.

The problem has to do with Safari or Webkit. See thread below.

https://discussions.apple.com/thread/252054628