Application Express :: IR Report Page With Download (specific) Column Security?

i have IR report page and download column as blob,how can security applied in specific column wise?e.g. Report page have more rows, and applied query in condition.

Select ID,dbms_lob.getlength(Blob_file) Download  from Document_master
where Created_by=UPPER(:APP_USER)
OR
  (exists (select ''   from   apex_workspace_group_users awgu
where  awgu.user_name =:app_user  AND awgu.GROUP_NAME='EMPLOYEE_GROUP'
)
)

Now all the rows with Download column to see EMPLOYEE_GROUP users,but i need control the download column only except Created_by=UPPER(:APP_USER) ,this case how can do the security.