An integer expression indicating the 1-based index of the attribute.
The attribute type is one of the following strings: "CDATA," "ID," "IDREF," "IDREFS," "NMTOKEN," "NMTOKENS," "ENTITY," "ENTITIES," or "NOTATION." These are always uppercase.
Returns the Unknown value (?) if the index is less than 1 or greater than the value of SAX-attributes object's NUM-ITEMS attribute.