Simplify/fix send_file last_modified logic
Simplify/fix send_file last_modified logic

- do not manually set it
- avoid converting to timestamp and back, which seems to subtract the
  local timezone's utc offset again

DirkHoffmann/indico,pferreir/indico,indico/indico,indico/indico,DirkHoffmann/indico,DirkHoffmann/indico,pferreir/indico,indico/indico,ThiefMaster/indico,DirkHoffmann/indico,ThiefMaster/indico,indico/indico,pferreir/indico,ThiefMaster/indico,pferreir/indico,ThiefMaster/indico