The only reason people think it's a Normal-type is because it looks like Audino, but it doesn't make any sense. The evolution of Eevee is susceptibility to the radiation around it, which comes from an evo stone or the environment. This way, how could Eevee change form and still be Normal? Normal is the "untouched" Eevee, with its DNA intact. If it changed form by evolving, it's no longer Normal.
I'll be very disappointed if it turns out to be a Normal-type, as the whole point of Eevee is to change type upon evolving... But I'm also 99% sure it won't be Normal-type. And I'm also a bit less sure it won't be Flying. Ask yourselves a question: how could it fly? I mean actually taking off by itself and flying. The absolute majority of the Flying-type Pokémon can take off by themselves. Birds and dragons flap their wings, Tropius uses its large leaves on its back, Mantine can glide over the surface of water after jumping out of it at high speed.
The only Flying-type Pokémon that needs the help of the wind to fly is the Hoppip line, but Hoppip and its evolutions weigh almost nothing by Pokémon standards, so they're light enough to float in the air and control their flight. Meanwhile, Sylveon weighs more than 20 kilos. He's the lightest of the eeveelutions, but he's not that much lighter than the rest, and not light enough to be caught by the wind, unless it's a hurricane.
Gyarados can't fly, but he's only Flying-type because of the legend of the carp.
If it turns out Sylveon is actually Flying, he'll have to be special biased, because most of the physical Flying-type moves are related to birds.