Incorrect Default Thumbnail for PDF Files

### ⚠️ Before posting ⚠️

- [x] This is a **bug**, not a question or an enhancement.
- [x] I've [searched for similar issues](https://github.com/nextcloud/android/issues) and didn't find a duplicate.
- [x] I've written a clear and descriptive title for this issue, not just "Bug" or "Crash".
- [x] I agree to follow Nextcloud's [Code of Conduct](https://nextcloud.com/contribute/code-of-conduct/).

### Steps to reproduce

1. Upload PDF files

### Expected behaviour

PDF files should display a correct thumbnail.

### Actual behaviour

Thumbnails for PDF files are folder icon.

### Android version

15

### Device brand and model

Android Emulator

### Stock or custom OS?

Stock

### Nextcloud android app version

3.31.0

### Nextcloud server version

31.0.0

### Android logs