APK signature .asc file extension is renamed to .key

The signature file here, when downloaded from a Linux desktop, is saved as F-Droid.apk.asc, as expected.

On my Android phone, it is renamed to F-Droid.apk.key on Firefox, and F-Droid.apk.asc.key on Chrome. The contents don't change, only the filename. It's just a minor inconvenience, as it results in incorrect "Open With" actions.

What do you think is causing this?