def create_category(session, displayname, file_extensions,
                    return_type=None, **kwargs):
    """
    Create a new file lifecycle category with its file extensions.

    :type session: zadarapy.session.Session
    :param session: A valid zadarapy.session.Session object.  Required.

    :type displayname: str
    :param displayname: The category displayname 'category_name' value as returned by
        get_all_categories.  For example: 'Video Files'.  Required.

    :type file_extensions: str
    :param file_extensions: The category file extensions 'extension' value as returned by
        get_all_categories.  For example: 'avi,mpg,mpeg'.  Required.

    :type return_type: str
    :param return_type: If this is set to the string 'json', this function
        will return a JSON string.  Otherwise, it will return a Python
        dictionary.  Optional (will return a Python dictionary by default).

    :rtype: dict, str
    :returns: A dictionary or JSON data set as a string depending on
        return_type parameter.
    """
    body_values = {'displayname': displayname, 'file_extensions': file_extensions}

    path = '/api/flc/create_category.json'

    return session.post_api(path=path, body=body_values,
                            return_type=return_type, **kwargs)