I've heard somewhere that Aristotle's reputation with medieval scholars was so high that nobody questioned him writing that files have 4 legs instead of 6 despite how easy it would be to verify. Is this anecdote true?

Aristotle wrote:

περὶ τὸν Ὕπανιν ποταμὸν τὸν περὶ Βόσπορον τὸν Κιμμέριαν, γίγνεται ζῶον πτερωτὸν, τετράπουν. ζῇ δὲ τοῦτο καὶ πέτεται ἐξ ἑωθινοῦ μέχρι δείλης· καταφερομένου δὲ τοῦ ἡλίου, ἀπομαραίνεται, καὶ ἅμα δυομένῳ ἀποθνήσκει, βιοῦν ἡμέραν μίαν· διὸ καὶ καλεῖται Ἐφήμερον.

which is, in the Thompson translation,

On the river Hypanis in the Cimmerian Bosphorus, about the time of the summer solstice, there are brought down towards the sea by the stream what look like little sacks rather bigger than grapes, out of which at their bursting issues a winged quadruped. The insect lives and flies about until the evening, but as the sun goes down it pines away, and dies at sunset having lived just one day, from which circumstance it is called the ephemeron.

Hypanis is the Southern Bug in Ukraine:

https://commons.wikimedia.org/wiki/File:PietinisBugas.png

(The Kuban River in the northern Caucasus was also called the Hypanis, but "the Cimmerian Bosphorus" makes it clear that the Southern Bug is intended.)

The "ephemeron" is not just any fly, but that most ephemeral of flies, the mayfly (aptly called "Eintagsfliege" ("one-day fly") in German). I'm not aware of Aristotle having visited Ukraine, so he might have just been reporting hearsay. Generally, Aristotle was quite aware that flies had six limbs, and indeed wrote about six-legged flies. So, why would he mention four-legged ("tetrapous") flies? Perhaps he looked at mayflies himself, and was satisfied that four-legged was a fair description, since many walk on four legs, keeping the front legs off the ground:

The four-legged ephemeron text is in Book V of Aristotle's History of Animals:

https://en.wikisource.org/wiki/History_of_Animals_(Thompson)

There is more discussion of insects in Book IV.