Technically, uploading the files for distribution would be illegal, yes. But if the company that owned the rights is defunct or doesn't care, there would be probably no consequences.

Archivists have a private (not available for public download) archive of game data backed up, which is perfectly legal, as long as you purchased or was given the game. I personally have a backup library of legitimately copied files of my cartridge games; useful for playing the games via emulation and keeping their physical condition good.