Inside the document folder of a project, create a new folder that is
access_internal='view' and access_via_link='none'. Inside that folder
create a document that is access_via_link='view'.

From the Documents application standpoint, you created a document that
cannot be discovered by a public user, unless he get the access-token to
that file.

But you can access the file via the Project's documents even without the
access token, because the conditional `res_id`, `res_model`,
`access_via_link` are met.

It means that if you're a logged-in portal user you won't see that file
as the `user_permission` is correctly `none`. But you can disconnect,
become a public user, and get access to the file.

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An explored solution was to create a dedicated "shared" folder for every
project/task, and to place all publicly discoverable files within that
folder. Files that should be shared on a link-only manner would be
placed elsewhere would not be listed but for the portal users.

This solution was rejected because it created too many folders.

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The solution here is to restrict accessing documents to logged-in users,
so that the single always-valid `user_permission` field can be used and
there's no need to re-engineer something for public users.