Which OS and web browser are you using? Do you have another web browser installed on your computer you can try using? ^GD
I saved it on my desktop and tried to drop the file in, that's where I was getting the error message. I saved it on another location on my computer and it worked fine. Using an HP Elite Book and Chrome.
I've been getting this error intermitantly from attendees too. I've tried renaming it to different names, but turns out the error is related to Chrome. In Chrome (and Chromium based browsers) it always seems to fail if there are commas in the filename. Works fine in Firefox and Edge, at least until Edge switches to Chromium engine.
Different computer operating systems allow/disallow certain characters in file names. Some web browsers know how to handle this and some do not.
It is best to use just letters, numbers, and the underscore _ character in file names. Spaces should be OK.