def get_invoice_filename(metadata_file_xml_root,
    metadata_file_invoice_filename_xml_tag: str,
    metadata_file_xml_namespace: str) ->str:
    """Return the file name of the invoice file.

    :param metadata_file_xml_root: the root of the metadata XML tree.
    :param metadata_file_invoice_filename_xml_tag: the tag name corresponding
        to the invoice filename.
    :param metadata_file_xml_namespace: the XML namespace of the metadata file.
    :type metadata_file_xml_root: lxml.etree._Element
    :type metadata_file_invoice_filename_xml_tag: str
    :type metadata_file_xml_namespace: str
    :returns: the element or ``None``, if no match is found.
    :rtype: str
    :raises: an lxml or a built-in exception.
    """
    return metadata_file_xml_root.find(metadata_file_invoice_filename_xml_tag,
        metadata_file_xml_namespace).text
