def allowed_file(filename, extensions={'csv'}):
    """
    Checks if a filename contains an allowable extension.

    Parameters
    ----------
    filename : str
        The filename to check.
    extensions : set
        The set of allowable file extensions.

    Returns
    -------
    allowed : bool
        True if allowable extension, False otherwise.
    """
    return '.' in filename and \
           filename.rsplit('.', 1)[1].lower() in extensions