Upwork is NOT a file transfer tool. It has never claimed to be.
As a professional, you will be very familiar with and use actual file transfer tools, because you know it will make things easier, save time, and you also know that sending files through Upwork can damage them.
You will use your favorite options among such tools as:
Microsoft One Drive
USB thumb drive
Call me old-fashioned, but FTP is still one of my favorite ways to get files to clients who aren't on Upwork.
Setting up your own server might not be the most user friendly thing in the world, but if you give them their own folder and account, you then know exactly when they finish downloading each file. So if there's ever claim they didn't get their files, I can simply say "they were downloaded at this time, by this IP, and you're the only one with the login information." Never had it come up, but better to have logs and not need them =)