harry potter - Were the flying keys charmed or transfigured?

So in the climax of The Philosopher's Stone, we see Harry and the Gang pass through some challenges to stop Quirell retrieving the stone; one of these challenges is chasing down a flying key. Conveniently, Hermione checks off the challenges and their progenitors:

“We’ve had Sprout’s, that was the Devil’s Snare – Flitwick must’ve put charms on the keys – McGonagall transfigured the chessmen to make them alive – that leaves Quirrell’s spell, and Snape’s…”

— Chapter 16, Through the Trapdoor

However, that the keys were charmed (and not transfigured) seems at odds with what we see of Charms and Transfiguration in the series. Charms are shown to change how objects behave, rather than adding to their appearance; as worded on the Harry Potter wikia:

Charms are distinguished from transfigurations in that a charm adds or changes properties of an object; it focuses on altering what the object does as opposed to what the object is.

On the other hand, Transfiguration is shown to add or transform one object into another, or change an object into a living creature.

Now, if the keys had been bewitched to float around, it would clearly be a charm. However, the keys are given wings, which, for me, blurs the line between a floating charm and a transfiguration (e.g. into a bird kind of thing). So, is there any evidence (apart from Hermione's conjecture) that giving the keys wings is an instance of charms rather than having been transfigured?

Answer

As you described, a Charm adds properties. If it requires it, the object may change its appearance by growing visible additions, such as arms, legs or indeed wings. For example, in Order of the Phoenix, Harry and Ron are shown bewitching legs onto teacups during a Charms lesson.

“Should we say something?” said Hermione in a worried voice, pressing her cheek against the Charms window [...]

He and Ron both tapped the teacups they were supposed to be charming with their wands. Harry’s spouted four very short legs that would not reach the desk and wriggled pointlessly in midair. Ron’s grew four very thin spindly legs that hoisted the cup off the desk with great difficulty, trembled for a few seconds, then folded, causing the cup to crack into two.

Harry Potter and The Order of the Phoenix, Chapter 30 Grawp

I think you're right that it blurs the line, but a Charm I think it definitely was. Flitwick also asked students to make a pineapple dance across a desk, and that may have required legs. Perhaps the distinction is that the keys and other items aren't turned into recognizable different objects.