code: def subjects_download(self, subject_id):
          
          # Retrieve subject to ensure that it exist
          subject = self.subjects_get(subject_id)
          if subject is None:
              # Return None if subject is unknown
              return None
          else:
              # Reference and information for original uploaded file
              return FileInfo(
                  subject.data_file,
                  subject.properties[datastore.PROPERTY_MIMETYPE],
                  subject.properties[datastore.PROPERTY_FILENAME]
              )
docs: Get data file for subject with given identifier.
  
          Parameters
          ----------
          subject_id : string
              Unique subject identifier
  
          Returns
          -------
          FileInfo
              Information about subject's data file on disk or None if identifier
              is unknown