> the fact that it scans for specific extensions sounds more like a product of an API limitation (i.e. no available getAllExtensions() or somesuch) vs. something inherently sinister (e.g. “they’re checking to see if you’re a Muslim”).

Your computer is your private domain. Your house is your private domain. You don't make a "getAllKeysOnPorch()" API, and certainly don't make "getAllBankAccounts()" API. And if you do, you certainly don't make it available to anyone who asks.

It absolutely is sinister.