gecko-dev/dom/fetch/tests/browser_blobFromFile.js

add_task(async function test() {
  await SpecialPowers.pushPrefEnv(
    {set: [["browser.tabs.remote.separateFileUriProcess", true]]}
  );

  let fileData = "";
  for (var i = 0; i < 100; ++i) {
    fileData += "hello world!";
  }

  let file = Cc["@mozilla.org/file/directory_service;1"]
               .getService(Ci.nsIDirectoryService)
               .QueryInterface(Ci.nsIProperties)
               .get("ProfD", Ci.nsIFile);
  file.append('file.txt');
  file.createUnique(Ci.nsIFile.NORMAL_FILE_TYPE, 0o600);

  let outStream = Cc["@mozilla.org/network/file-output-stream;1"]
                      .createInstance(Ci.nsIFileOutputStream);
  outStream.init(file, 0x02 | 0x08 | 0x20, // write, create, truncate
                 0666, 0);
  outStream.write(fileData, fileData.length);
  outStream.close();

  let fileHandler = Cc["@mozilla.org/network/io-service;1"]
                      .getService(Ci.nsIIOService)
                      .getProtocolHandler("file")
                      .QueryInterface(Ci.nsIFileProtocolHandler);

  let fileURL = fileHandler.getURLSpecFromFile(file);

  info("Opening url: " + fileURL);
  let tab = BrowserTestUtils.addTab(gBrowser, fileURL);

  let browser = gBrowser.getBrowserForTab(tab);
  await BrowserTestUtils.browserLoaded(browser);

  let blob = await ContentTask.spawn(browser, file.leafName, function(fileName) {
    return new content.window.Promise(resolve => {
      content.window.fetch(fileName).then(r => r.blob()).then(blob => resolve(blob));
    });
  });

  ok(blob instanceof File, "We have a file");

  is(blob.size, file.fileSize, "The size matches");
  is(blob.name, file.leafName, "The name is correct");

  file.remove(false);

  gBrowser.removeTab(tab);
});