docs: def pyfs_storage_factory(fileinstance=None, default_location=None,
                           default_storage_class=None,
                           filestorage_class=PyFSFileStorage, fileurl=None,
                           size=None, modified=None, clean_dir=True):
      """Get factory function for creating a PyFS file storage instance."""
      # Either the FileInstance needs to be specified or all filestorage
      # class parameters need to be specified
      assert fileinstance or (fileurl and size)
  
      if fileinstance:
          # FIXME: Code here should be refactored since it assumes a lot on the
          # directory structure where the file instances are written
          fileurl = None
          size = fileinstance.size
          modified = fileinstance.updated
  
          if fileinstance.uri:
              # Use already existing URL.
              fileurl = fileinstance.uri
          else:
              assert default_location
              # Generate a new URL.
              fileurl = make_path(
                  default_location,
                  str(fileinstance.id),
                  'data',
                  current_app.config['FILES_REST_STORAGE_PATH_DIMENSIONS'],
                  current_app.config['FILES_REST_STORAGE_PATH_SPLIT_LENGTH'],
              )
  
      return filestorage_class(
          fileurl, size=size, modified=modified, clean_dir=clean_dir)