get extension() {
    if (typeof this._extension === 'string') {
      return this._extension;
    } else {
      return this.defaultExtension;
    }
  }
The file name extension for the asset (String). It is
automatically kept in sync with the url, but preserved if the
asset is inlined or set to a value that ends with a slash.

If updated, the url of the asset will also be updated.

The extension includes the leading dot and is thus kept in the
same format as `require('path').extname` and the `basename`
command line utility use.

@type {String}