Gets the last component of the filename.
file_name ends with a directory separator it gets the component
before the last slash. If
file_name consists only of directory
separators (and on Windows, possibly a drive letter), a single
separator is returned. If
file_name is empty, it gets “.”.
The name of the file.
The data is owned by the caller of the function. The value is a file system path, using the OS encoding.