@arev There is a way to get 2.8 files it into Sketchfab, but it’s a workaround and I found that it doesn’t always work well.
I think the better and easiest way would be to select your object and copy it with Ctrl+C, then paste it in a new file in 2.79 with Ctrl+V, and then save it. Then you can upload the 2.79 file with ease. Make sure all modifiers are applied (if you have any), because they won’t transfer (well).
If it doesn’t work for you I wouldn’t worry too much, @theluthier did change it to just pictures for a reason 😊