def _ParseExtensionsString(self, extensions_string):
    """Parses the extensions string.

    Args:
      extensions_string (str): comma separated extensions to filter.
    """
    if not extensions_string:
      return

    extensions_string = extensions_string.lower()
    extensions = [
        extension.strip() for extension in extensions_string.split(',')]
    file_entry_filter = file_entry_filters.ExtensionsFileEntryFilter(extensions)
    self._filter_collection.AddFilter(file_entry_filter)