_getExtension(str) {
	    if (!str) {
	      return '';
	    }

	    const basename = path.basename(str);
	    const firstDot = basename.indexOf('.');
	    const lastDot = basename.lastIndexOf('.');
	    const extname = path.extname(basename).replace(/(\.[a-z0-9]+).*/i, '$1');

	    if (firstDot === lastDot) {
	      return extname;
	    }

	    return basename.slice(firstDot, lastDot) + extname;
	  }
Get case-insensitive `field` value.

@param {String} field
@return {String}
@api public