Integration with fof/pages: File rendering & download should work outside of original post

[JeromeGillard]

Bug Report

Current Behavior
Uploaded files are not rendered in fof/pages.
Uploaded files cannot be downloaded fof/pages.

Steps to Reproduce

• create a post, let's say it has route /d/1-test
• upload a file and insert it as bbcode in the post: [upl-file uuid=e737f663-f38f-4d93-9d1c-5ec7abd4b19c size=2kB url=https://www.test.com/assets/files/2022-10-12/1665582177-619189-tes.gpx]test.gpx[/upl-file]
• create a fof/page , let's say it has route /p/2-new-page
• paste the same bbcode in the page
• error: the file cannot be displayed nor downloaded

Expected Behavior
The bbcode [upl-file uuid=e737f663-f38f-4d93-9d1c-5ec7abd4b19c size=2kB url=https://www.test.com/assets/files/2022-10-12/1665582177-619189-tes.gpx]test.gpx[/upl-file] should be working on pages.

Environment

• Flarum version: 1.3.0
• Extension version: 1.2.3

Possible solution(s)

A workaround is to create a post that has the same post.id than the page route's. Then it works.
But the page existence is now tied to another public post.

Additional Context

• ideally, a file should not depend on a post's existence
• having a uuid in the bbcode should make size & url redundant.
• having url in the bbcode is a big problem for domain name portability
• Having the rendering to fetch the file to yet another URL like /api/fof/download/e737f663-f38f-4d93-9d1c-5ec7abd4b19c/2-new-page/2A2AZKCJO8dHR907FnyRBQBf2by07jKUzNVFrxo0 is odd. Why include the current route instead of the original one?