How to check if an argument is a file or a directory in R?

I am quite familiar with the python equivalents 'os.path.isfile(path)' and 'os.path.isdir(path)' which say if path is a file or a directory.

So far, I have failed to find a simple and straightforward way to do this in R and it seems that even Google has never heard about this either !

Does anybody can beat Google and tell me what is the simplest way in R just to check if a given argument is a file, a directory or none of them ?