iOS 27: notification image thumbnail not showing after 2026.9.1 update

### The problem

After updating the Home Assistant Companion app to **2026.9.1** today, notification image thumbnails stopped appearing on **iOS 27**. The same payload worked fine earlier today with the previous app version, and still works perfectly on an **iOS 18** device using the same HA server.

**Symptoms:**
- Push notification arrives ✓
- Full-size image appears when the notification is **expanded** ✓
- Small **thumbnail** (top-right corner) is **missing** ✗
- No error in HA logs, no error in the app

### What version of Home Assistant Core has the issue?

2026.9.x (frontend: current)

### What was the last working version of Home Assistant Core?

Previous Companion app version (worked this morning, before the 2026.9.1 auto-update)

### What type of installation are you running?

Home Assistant OS

### Integration causing the issue

Mobile App (iOS Companion)

### Link to integration documentation on our website

https://companion.home-assistant.io/docs/notifications/notifications-basic

### Diagnostics information

**iPhone (issue device):**
- iOS version: 27.x
- HA Companion app version: 2026.9.1
- Installed: auto-update today
- Result: **no thumbnail**

**Second iPhone (control device):**
- iOS version: 18.x
- HA Companion app version: same 2026.9.1
- Result: **thumbnail works fine**

**Same HA server, same payload, same network.**

### Example YAML snippet

```yaml
service: notify.mobile_app_XXXXX
data:
title: "TEST"
message: "Test with image"
data:
attachment:
url: "https://example.duckdns.org/local/images/test.png"
content-type: png
hide-thumbnail: false

Start by reproducing the YAML notification payload on iOS 27 with Home Assistant Companion 2026.9.1, then compare it with the working iOS 18 device using the same server and payload. Use the linked notification documentation as context. Done means the push still arrives and the image appears both as an expanded image and as a small thumbnail on iOS 27.

Written by the indexing model from the issue text.