Preview of a link shared (without password) file can be downloaded using random filename

## Steps to reproduce
1. Create a link shared (without password) of a file (`lorem.txt`)
2. Download the preview of the shared file (using random name)
```bash
curl 'https://localhost:9200/remote.php/dav/public-files/OwdLARLBqcmKxgG/random-filename?preview=1' -vk
```
```bash
< HTTP/1.1 200 OK
< Content-Type: image/jpeg
```

Start by reproducing the public DAV preview request from the issue with the curl command, then trace the Go entry point handling shared-file previews and random filenames. Done means the documented request no longer downloads the preview for an unrelated filename while valid shared-file preview behavior remains intact.

Written by the indexing model from the issue text.