 In addition, the classic Mac OS did not recognize file extensions at all, leading to unrecognized file errors when files were transferred from DOS/Windows systems.OPENSTEP, which formed the basis of Mac OS X, used extensions, and early versions of Mac OS X followed suit.This led to some controversy with users and developers coming to OS X from NeXT or Windows origins advocating for continued use of file extensions, and those coming from Classic Mac OS urging Apple to replace or supplement file extensions with type and creators.