start.js

const puppeteer = require('puppeteer');

(async () => {
  const browser = await puppeteer.launch({ headless: false });

  const context = browser.defaultBrowserContext();
  await context.overridePermissions('https://stretch3.github.io', ['camera', 'microphone']);

  const page = (await browser.pages())[0];
  await page.goto('https://stretch3.github.io/');

  const uploadfile = '/home/pi/robot/scratch3-autostart/main.sb3';

  const fileMenu = (await page.$x("//*[contains(text(), 'ファイル')]"))[0];
  await fileMenu.click()
  const loadMenu = (await page.$x("//*[contains(text(), 'コンピューターから読み込む')]"))[0];

  const [fileChooser] = await Promise.all([
    page.waitForFileChooser(),
    loadMenu.click(),
  ]);
  await fileChooser.accept([uploadfile]);

  await page.bringToFront();
  await page.waitForTimeout(2000);

  const executeButton = await page.$('[title="実行"]');
  await executeButton.click();
})();