I understand your example:
ftp.example.com/foobar
^^^here, foobar would be a text file
ftp.example.com/foobar/myfile.zip
^^^here, foobar would be a directory

but the lack of an extension doesn't imply a directory.   I could just as
easily say:

ftp.example.com/foobar.txt
^^^here, foobar.txt would be a text file
ftp.example.com/foobar.txt/myfile.zip
^^^here, foobar.txt would be a directory

so why are non extension files treated any differently?