2. Import source (File or Relational)
3. Creation a simple mapping.
Source --> Expression (to specify the filename) --> Target
In the expression you specify below as a filename'HelloWorld_' || TO_CHAR(SYSTIMESTAMP(),'DD-MM-YYYY')
4. Create a workflow and specify target directory and target filename at session properties level.
5. file name which you give at session level will get over-ridden by file name which you will pass from mapping.