PreviewExtension/preview.html

<!DOCTYPE html>
<!-- This Source Code Form is subject to the terms of the Mozilla Public
   - License, v. 2.0. If a copy of the MPL was not distributed with this
   - file, You can obtain one at https://mozilla.org/MPL/2.0/. -->
<html>
  <head>
    <meta charset="utf-8" />
    <style>
      * {
        box-sizing: border-box;
      }
      #preview {
        text-align: center;
        font-family: system-ui;
        font-size: 24px;
        margin: 4em;
        word-wrap: break-word;
      }
    </style>
  </head>
  <body>
    <div id="preview"></div>
    <script>
      async function main() {
        const { file, path } = await quicklook.getPreviewedFile();
        const contents = await file.text();

        document.getElementById(
          "preview"
        ).innerText = `File contents are: “${contents.trim()}”\n\nPath is: “${path}”`;

        await quicklook.finishedLoading();
      }
      main();
    </script>
  </body>
</html>