docs.sheetjs.com/docz/static/next/next.config.js

module.exports = {
  webpack: (config) => {
    /* add to the webpack config module.rules array */
    config.module.rules.push({
      /* `test` matches file extensions */
      test: /\.(numbers|xls|xlsx|xlsb)/,
      /* use the loader script */
      use: [ { loader: './base64-loader' } ]
    });
    return config;
  }
};