Clicking internal link to uploaded file redirects to home instead of opening it #5012

Current Behavior

Bug Report

Current Behavior

Clicking on a direct in-post link to an uploaded file (e.g. an image link, <a href="https://example.com/assets/files/...jpg" class="UrlLink UrlLink--internal">) redirects the user to the forum's home page (/) instead of navigating to the file.

Opening the same link in a new tab (e.g. via "Open in new tab") works correctly and loads the image as expected — the issue only occurs on a direct in-app left-click.

Steps to Reproduce

1. Upload an image to a post (via FoF Upload or core upload)
2. The resulting link in the rendered post has class UrlLink UrlLink--internal
3. Left-click directly on the image/link within the post
4. Observe: instead of opening/viewing the image, the app redirects to the forum home page
5. Right-click → "Open in new tab" on the same link works fine and loads the image directly

Expected Behavior

Clicking the link should either:

• Load/display the linked file (image), or
• At minimum, not redirect to an unrelated page (home)

Root Cause (found via investigation)

• Formatter::isInternalUrl() in vendor/flarum/core/src/Formatter/Formatter.php marks any same-host URL as internal (UrlLink--internal), including links to static uploaded files that have no corresponding Mithril route (e.g. /assets/files/...).
• The client-side handler in js/src/forum/utils/routeInternalLinks.ts trusts this class without checking whether the path actually corresponds to a registered app route.
• Since /assets/files/... doesn't match any registered Mithril route, m.route.set() silently falls back to the default route (home), per the same mechanism described in the code's own comment about forums installed in a subdirectory.

Context

Steps to Reproduce

1. Upload an image to a post (e.g. via FoF Upload or core upload)
2. The resulting link in the rendered post has class UrlLink UrlLink--internal
3. Left-click directly on the image/link within the post
4. Observe that instead of opening/viewing the image, the app redirects to the forum home page
5. Right-click → "Open in new tab" on the same link works fine and loads the image directly

Expected Behavior

Clicking the link should either load/display the linked file (image), or at minimum not redirect to an unrelated page (home).

Screenshots

No response

Environment

• Flarum version: x.y.z
• Website URL: http://example.com
• Webserver: [e.g. apache, nginx]
• Hosting environment: [e.g. shared, vps]
• PHP version: x.y.z
• Database: [e.g. MySQL 8.4.11, MariaDB 10.11.2, PostgreSQL 16.1, SQLite 3.45]
• Browser: [e.g. chrome 67, safari 11]

Output of php flarum info

Output of "php flarum info", run this in terminal in your Flarum directory.

Possible Solution

No response

Additional Context

No response