Came over this thread on TechCommunity. https://techcommunity.microsoft.com/t5/Skype-for-Business-IT-Pro/New-Skype-for-Business-on-Mac-Insider-build-is-available/m-p/60378#M1547

On Thursday last week they published a new build of the Skype for Business Mac client on the Insider Fast program. File sending is now in beta after a huge amount of requests from the community.

You can activate the Insider Fast program by opening Skype for Business Mac, then press help and Check for updates.

Then set it like this (Sorry for the Norwegian text 😉 ):