_getFile() {
    const ext = path.extname(this._basename);
    const basename = path.basename(this._basename, ext);
    const isRotation = this.rotationFormat
      ? this.rotationFormat()
      : this._created;

    // Caveat emptor (indexzero): rotationFormat() was broken by design When
    // combined with max files because the set of files to unlink is never
    // stored.
    return !this.tailable && this._created
      ? `${basename}${isRotation}${ext}`
      : `${basename}${ext}`;
  }
Gets the next filename to use for this instance in the case that log
filesizes are being capped.
@returns {string} - TODO: add return description.
@private