Hints and tricks for Oracle and other stuff...

 function ConvertBlobXMLToClob(pfile documents.name%type) return clob is
   type message_type is record (
        content blob,
        charset varchar2(30),
        typ varchar2(20)
   );
   v_message message_type;
   v_content clob;
   v_dest_offset number := 1;
   v_src_offset number := 1;
   v_lang_context number := dbms_lob.default_lang_ctx;
   v_warning number;
   v_x1 number;
 begin
   -- preberimo dokument
   select t.blob_content, 'UTF8' --t.dad_charset
        , t.mime_type
       into v_message.content, v_message.charset, v_message.typ
   from documents t
   where t.name = pfile;

   -- to CLOB
      dbms_lob.createtemporary(v_content, TRUE);
      v_dest_offset := 1;
      v_src_offset  := 1;
      v_lang_context := dbms_lob.default_lang_ctx;
      DBMS_LOB.CONVERTTOCLOB(v_content, v_message.content, dbms_lob.getlength  
      (v_message.content), v_dest_offset, v_src_offset,
      nls_charset_id(v_message.charset), v_lang_context, v_warning);
  
      v_x1 := instr(lower(substr(v_content,1,1000)),'encoding="');

      if v_x1 > 0 then
        v_message.charset := replace(substr(v_content,v_x1+10,instr(lower
        (substr(v_content,1,1000)),'"', v_x1+10)-v_x1-10),'-','');

        dbms_lob.createtemporary(v_content, TRUE);
        v_dest_offset := 1;
        v_src_offset  := 1;
        v_lang_context := dbms_lob.default_lang_ctx;
        DBMS_LOB.CONVERTTOCLOB(v_content, v_message.content,
        dbms_lob.getlength(v_message.content), v_dest_offset, v_src_offset,
        nls_charset_id(v_message.charset), v_lang_context, v_warning);
      end if;                         
  
   return v_content;   
 end;