Parse XML file which in BLOB data type via XPATH

How to parse XML file which in BLOB data type via XPATH in Oracle 10g

The following function can parse the XML file which stored in Oracle BLOB type column via XPATH.
The function has 2 input parameters:
V_ATTACHMENT:  This  parameter should pass in the BLOB column, if the column is in CLOB type, you can change the function, set the data type to CLOB and modify the 1st query SELECT FN_BLOB2CLOB(v_ATTACHMENT) INTO v_CLOB FROM DUAL to SELECT v_ATTACHMENT INTO v_CLOB FROM DUAL
V_EXPRESSION:  This parameter is to pass in the XPATH Expression.
The function has 1 return value in VARCHAR2 type.
CREATE OR REPLACE FUNCTION DMTA.FN_PARSE_XPATH(v_ATTACHMENT IN BLOB, v_EXPRESSION IN VARCHAR2)
RETURN VARCHAR2 IS

v_XML   XMLType;
v_XML2  XMLType;
v_CLOB  CLOB;


BEGIN

    SELECT FN_BLOB2CLOB(v_ATTACHMENT) INTO v_CLOB FROM DUAL;

    v_XML := sys.XMLType.createXML(v_CLOB);

    SELECT EXTRACT(v_XML,v_EXPRESSION) INTO v_XML FROM DUAL;

    RETURN v_XML.getStringVal();

   EXCEPTION
     WHEN NO_DATA_FOUND THEN
       NULL;
     WHEN OTHERS THEN
       RETURN NULL;

END;