LFS: Error 500 when trying to view new LFS files in a PR from a fork

[w23]

Description

1. Have a repo that has LFS enabled.
2. Make a fork, commit a bunch of new LFS files into the fork.
3. Make a PR against the original repo.
4. Click on View File button on any of the new LFS files in the PR.
5. See error 500 instead of the preview. Logs and the error message say
   ...ers/web/repo/view.go:342:renderFile() [E] getFileReader: LFS Meta object does not exist

More observations:

• Other than that LFS works fine for both repos.
• Clicking on the same file when viewing the forked repo works just fine.
• View file button on both PR and forked repo commit leads to almost the same URL, with the only difference being the repo owner part.
• After merging the PR the issue disappears, i.e. the view file URL that has upstream repo owner part no longer leads to error 500.

Also, it would be super nice to have the same image comparison/difference preview for LFS files in a PR, as for regular image files.

Gitea Version

1.21.1

Can you reproduce the bug on the Gitea demo site?

No

Log Gist

No response

Screenshots

No response

Git Version

No response

Operating System

No response

How are you running Gitea?

gitea/gitea:latest docker image with the most basic setup and essentially default settings.

Database

None

[s-weigand]

I think this is a duplicate of #17715
The workaround I use is going to the repos LFS settings, click the Find pointers files and after that associating them with the repos. Note that this also needs to be done for forks after it got merged upstream.