OS10.12 with Safari 10.0
When adding attachments to my email I can only select one file at a time. Previously I could upload multiple attachments at the same time.
I'm not sure if this is something you are working on or not.
Welcome to the community!
I'm not aware of any recent updates within any of our email services that would have prevented the upload of multiple file attachments. Have you tried reviewing with our live support team or using another browser to attempt the uploads?
Let us know if you manage to work this out.