_checkMaxFilesIncrementing(ext, basename, callback) {
    // Check for maxFiles option and delete file.
    if (!this.maxFiles || this._created < this.maxFiles) {
      return setImmediate(callback);
    }

    const oldest = this._created - this.maxFiles;
    const isOldest = oldest !== 0 ? oldest : '';
    const isZipped = this.zippedArchive ? '.gz' : '';
    const filePath = `${basename}${isOldest}${ext}${isZipped}`;
    const target = path.join(this.dirname, filePath);

    fs.unlink(target, callback);
  }
Increment the number of files created or checked by this instance.
@param {mixed} ext - TODO: add param description.
@param {mixed} basename - TODO: add param description.
@param {mixed} callback - TODO: add param description.
@returns {undefined}
@private