[Bug]: Thumbnails are being requested for folders

### ⚠️ Before submitting, please verify the following: ⚠️

- [x] This is a **bug**, not a question or a configuration issue.
- [x] This issue is **not** already reported on Github (I've searched it).
- [x] Nextcloud Server and Desktop Client are **up to date**. See [Server Maintenance and Release Schedule](https://github.com/nextcloud/server/wiki/Maintenance-and-Release-Schedule) and [Desktop Releases](https://nextcloud.com/install/#install-clients) for supported versions.
- [x] I agree to follow Nextcloud's [Code of Conduct](https://nextcloud.com/contribute/code-of-conduct/)

### Bug description

HTTP GET requests are being made needlessly to "index.php/core/preview" for retrieving thumbnails for **folders**.

### Steps to reproduce

Browse a synched folder via Windows Explorer. Look for HTTP requests made.

### Expected behavior

ThumbnailProvider::GetThumbnail should:

`
SFGAOF attributes = 0;
if (SUCCEEDED(_shellItem->GetAttributes(SFGAO_FOLDER, &attributes)) && (attributes & SFGAO_FOLDER)) {
return E_FAIL;
}
`

### Which files are affected by this bug

None

### Operating system

Windows

### Which version of the operating system you are running.

Windows 11

### Package

Official Linux AppImage

### Nextcloud Server version

Latest

### Nextcloud Desktop Client version

Latest

### Is this bug present after an update or on a fresh install?

Updated from a minor version (ex. 3.16.1 to 3.16.2)

### Are you using the Nextcloud Server Encryption module?

Encryption is Disabled

### Are you using an external user-backend?

- [ ] Default internal user-backend
- [ ] LDAP/ Active Directory
- [ ] SSO - SAML
- [ ] Other

### Nextcloud Server logs

```shell

```

### Additional info

_No response_