code: def update_links_and_ffts(self):
          
          for field in record_get_field_instances(self.record,
                                                  tag='856',
                                                  ind1='4'):
              subs = field_get_subfields(field)
              newsubs = []
              url = subs.get("u", [])
  
              if not url:
                  record_delete_field(self.record, '856', ind1='4',
                                      field_position_global=field[4])
                  continue
              url = url[0]
              if "inspirehep.net/record" in url and url.endswith("pdf"):
                  # We have an FFT from INSPIRE
                  newsubs.append(('a', url))
                  description = subs.get("y", [])
                  if description:
                      newsubs.append(('d', description[0]))
                  if newsubs:
                      record_add_field(self.record, 'FFT', subfields=newsubs)
                      record_delete_field(self.record, '856', ind1='4',
                                          field_position_global=field[4])
              else:
                  # Remove $w
                  for idx, (key, value) in enumerate(field[0]):
                      if key == 'w':
                          del field[0][idx]
docs: FFT (856) Dealing with files.