First git commit says
Author: Brion Vibber <email@example.com> Date: Tue Jul 14 21:49:50 2009 +0000 Commit little UploadBlacklist extension we've had live for a few years but apparently forgot to commit. ;) It's pretty hacky, requiring sysadmins to add SHA-1 hashes to an array (haha! yeah that's scalable) Should get rewritten with a DB and a UI and integrated to core at some point.
I think I wrote this yes... we'd consider this as GPLv2-or-later as part of MediaWiki. It wasn't originally labeled in detail as it was a one-off production hack that ended up being kept around, and we weren't as careful with explicit labeling on extensions back then.