On nls charachter AL32UTF8 i'm trying to open files in Hebrew and they are not been recognized

On nls charachter AL32UTF8 i'm trying to open files in Hebrew and they are not been recognized (in other nls like utf8, IW8ISO8859P8 it's working fine, in English its working fine): 

why it happns and how can i solve it?  

declare       

    WR_BUFFER                    RAW(32767):='';

    WB_EXISTS                    BOOLEAN;

    WN_FILE_LENGTH               NUMBER;

    WN_BLOCKSIZE                 NUMBER; 

    l_bfile            BFILE; 

    AA VARCHAR2(1000):='דבאג.log';--CONVERT(TO_SINGLE_BYTE('בדיקה.txt'),'UTF8');          

begin

        utl_file.fgetattr  -- Use fgetattr to get the file length.

        (  

          'MAIL_DIR',

         'דבאג.log',

           WB_EXISTS,

           WN_FILE_LENGTH,

           WN_BLOCKSIZE

        );

dbms_output.put_line('WN_FILE_LENGTH: '|| WN_FILE_LENGTH);