Getting HTTP 302 -> /403 on displaying uploaded images after answer "update"

## Describe the bug

Not sure when it started (maybe with 1.2.1 or after running 1.2.1 for some days, but not sure), and have no idea how to analyze the following issue:

Uploading images works well, they're written to the `updates` folder in the file system.
But when displaying them, answer returns a HTTP 302 redirecting to `/403`.

Uploaded avatars and the brand image are still working.

There are no additional information in debug log, pardon me, I've no idea how to taggle this.

The installation is private, when changing it to public, images are shown. so it seems to be related to login session handling for images downloads.

### To Reproduce

Steps to reproduce the behavior:

1. having a private answer
1. deploy a rebuild answer binary (`answer build --with-plugins...`)
1. uploaded images result int http 302 -> /403 
1. clear cookie `visit` and reload page
1. uploaded images work again

### Expected behavior

Uploaded images continue to being shown also after answer updates

### Screenshots

If applicable, add screenshots or video to help explain your problem.

### Platform

- Device: Any
- OS: Official answer container image
- Version: v1.2.1 `--with-plugins`

ソースファイルもテストも指定されていません。Answer バイナリを再ビルドしてプライベートインストール上で問題を再現し、その後、画像のダウンロードパスとログインセッションの処理を調査してください。更新後も visit cookie を消去せずにアップロードした画像へアクセスできれば完了です。

索引モデルが issue の本文から書いたものです。