Accessing Uploaded Attachments and Images Return HTTP 302 with Location /403

## Describe the bug

Add images and attachment to a post successfully, no error shows up, and then reopen the post, but image isn't displayed and attachement can't be downloaded.

I inspect the network activities. Refreshing the post with the image sends HTTP GET, but receives HTTP 302 response with location /403.

Clicking the attachment sends the HTTP GET, but receives HTTP 302 response with location /403.

### To Reproduce

Steps to reproduce the behavior:

1. Unpack binary distribution from github, to directory ~/apache-answer-1.4.5-bin-linux-amd64, where there is the executable.
2. Follow the guide on the official site.
3. Run the command and set up database, email and admin.

`ubuntu@VM-0-8-ubuntu:~/apache-answer-1.4.5-bin-linux-amd64$ sudo ./answer init -C ./answer-data/`

4. Run the command to bring up the forum.

`ubuntu@VM-0-8-ubuntu:~/apache-answer-1.4.5-bin-linux-amd64$ sudo nohup ./answer run -C ./answer-data/`

5. Login, create a new question, post an image or upload an attachment.
6. Go back to landing page and reopen the previous post to examine the image and attachement, can't be shown or downloaded.
7. Inspect the network activities with dev tool and locate the URL:

`http://plusai.site/uploads/files/post/5smfTQVm1EU/test.pdf`
`http://plusai.site/uploads/post/5smt4sbUZA9.png`

8. On server, navigate to the data directory, which is

`~/apache-answer-1.4.5-bin-linux-amd64/answer-data`

The path to previous uploaded files are

`~/apache-answer-1.4.5-bin-linux-amd64/answer-data/uploads/files/post/5smfTQVm1EU.pdf`
`~/apache-answer-1.4.5-bin-linux-amd64/answer-data/uploads/post/5smt4sbUZA9.png`

9. confi.yaml after initialization is like
`
ui:
base_url: ""
api_base_url: ""
`

There is **_no public_url arttribute_**.

### Expected behavior

Image is displayed, and attachment can be downloaded by clicking the link.

### Screenshots

No screenshot.

### Platform

- Device: Server
- OS: Ubuntu 2024.5
- Browser and version: Edge Lastest
- Version: 1.4.5

Reproduce the issue with the 1.4.5 binary using `answer init -C ./answer-data/` and `answer run -C ./answer-data/`, then inspect the upload URLs, redirects, `confi.yaml`, and files under `answer-data/uploads/`. Trace why image and attachment requests redirect to `/403`; done means post images display and attachments download successfully.

Written by the indexing model from the issue text.