Renaming of uploaded files has been disabled for the last couple of months pending software updates; as of today it is now re-enabled for admins on all Wikimedia sites.

Local admins can now rename files as well as deleting them; this makes it easier to track history for images and other media that need to be moved to another name.

— brion

Update 00:49 UTC 22 Sep: We’ve tracked down a bug which at least on en.wikipedia would cause the redirected old name to stop working for 24 hours. Now fixed; notes for cause and workaround on enwp Village Pump.