sharing: Doesn't work with all files and directories

Reference: https://discuss.freedombox.org/t/sharing-app-with-tor-address/466

The Sharing app currently gives no indication of its limitations about sharing files and directories. There are multiple failure scenarios:

1. Sharing /tmp works but not a newly created directory /tmp/media with new files.
2. Sharing /var/log works but most of the files and directories are missing. Some of the files throw 403 Forbidden errors.
3. If a directory as a whole is not accessible like the /tmp/media case above, a 404 is thrown. Changing ownership, file permissions etc. has no effect. Nor does restarting the Apache server.
4. I was able to successfully add '/var/lib/ejabberd' as a share but got a 403 Forbidden when trying to access it.

Tested on FreedomBox development vagrant machine. Plinth is running as user plinth.