Add page types for file API pages

[wbamberg]

Part of #16255. Add page types for some new pages under the File API.

[github-actions]

Preview URLs

• https://pr16985.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/FileList/item
• https://pr16985.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/FileList/length
• https://pr16985.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/File_API/Using_files_from_web_applications
• https://pr16985.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/File_API

Flaws

None! 🎉

External URLs

URL: /en-US/docs/Web/API/FileList/length
Title: FileList.length
on GitHub

No new external URLs

---

URL: /en-US/docs/Web/API/FileList/item
Title: FileList.item()
on GitHub

No new external URLs

---

URL: /en-US/docs/Web/API/File_API
Title: File API
on GitHub

No new external URLs

---

URL: /en-US/docs/Web/API/File_API/Using_files_from_web_applications
Title: Using files from web applications
on GitHub

No new external URLs

[teoli2003]

👍