def ext_from_url(url):
    """
    Get the file extension from the given URL. Looks at the last part of the URL
    path, and returns the string after the last dot.

    :param url: the URL to the file whose extension is being determined
    :returns: the file extension or ``None``
    """
    file_name = url.split("/")[-1]
    if "." in file_name:
        ext = file_name.split(".")[-1]
        return f".{ext}"
    else:
        return None