Desktop settings plays Event Alerts sound on open

### What Happened?

Opening `System Settings → Desktop` immediately plays the "Event Alerts" sound ("system can’t do something in response to input"), even without any user action in that panel.

This happens because wallpaper directory scanning calls `add_wallpaper_from_file()` for every file, and invalid MIME types now trigger `Gdk.Display.get_default().beep()`.

This behavior appears to have been introduced in [commit c9888015](https://github.com/elementary/settings-desktop/commit/c9888015), where scan and user-import paths were unified, and the invalid-file beep path became shared.

### Steps to Reproduce

1. Ensure `Sounds → Event Alerts → Play sound` is enabled.
2. Place a non-image file in a scanned wallpapers directory, e.g. `/usr/share/backgrounds/cosmos/background-1.xml` (text/XML file).
3. Open `System Settings → Desktop`.
4. Observe the alert sound immediately on panel open.

(Confirmed locally: moving that file out of scanned directories stops the sound. Restoring it brings the sound back.)

### Expected Behavior

Opening the Desktop panel should not play an alert sound during automatic wallpaper directory scanning.

Invalid files found during background scan should be skipped silently (or logged), while alert sounds should be reserved for invalid user-triggered actions (e.g. importing/drag-dropping unsupported files).

### OS Version

8.x (Circe)

### OS Architecture

amd64 (on most hardwares)

### Session Type

Secure Session (Wayland, This is the default)

### Software Version

Latest release (I have run all updates)

### Log Output

```shell

```

### Hardware Info

_No response_