Reader: open and view non-media files from a private site

[hypest]

Expected behavior

When reading a post on a private site, one should be able to tap on a file link (say, a PDF file) and open the file on a suitable viewer in the system.

Actual behavior

The app delegates the file opening to the system Intents mechanism but that fails because the file is behind a private site and cookies/auth is not shared.

Steps to reproduce the behavior

1. On the web, create a post on a private site and add a PDF file to the post (you can use the Media Library for that)
2. On the web, like the post so you can easily find it in the mobile Reader
3. On the mobile Reader, go to the "My Likes" drop-down option, look for your new post and tap to open it
4. While viewing the post in the Reader, tap on the PDF link. The system will try to open the file in a PDF viewer app and fail with The file could not be downloaded. Please save the file to your local storage and then try to open it or similar, outside the app.

Tested on [device], Android [version]

Samsung Galaxy S7E, Android 8.0, app version 10.6