def flatten_multiindexed_columns(df):
    """Remove multi-indexing of multi-indexed column headers.

    The first layer is the "DataType", while the second layer is the "Format".
    This second layer will become a new row.

    Parameters
    ----------
    df : pandas.DataFrame
        DataFrame with two header levels: "Datatype" and "Format".

    Returns
    -------
    df : pandas.DataFrame
        DataFrame with the second header level ("Format") converted to a
        normal row.
    """
    # Flatten multi-index
    vals = df.index.tolist()
    df.loc["Format"] = df.columns.get_level_values(1)
    df.columns = df.columns.get_level_values(0)
    df = df.loc[["Format"] + vals]
    df.index.name = "Entity"
    df = df.drop(columns=["DataType"])
    return df