A new trailer has been released for the upcoming movie, and near the end of the video (starting at 1:21) it shows Sylveon in its animated form. Not to discuss the short itself, but the location and Sylveon's surrounding in the video seem to suggest it as a Flying type. Not to mention, the actual location seems to be
atop a very large and tall tree!! That could be the biggest clue of all given that Eevee evolves to adapt to its surroundings.
...or maybe that's just what I want it to suggest in my head. I don't know. :x