Cannot create new document due to extension error

### ⚠️ 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. click +
2. Create new document
3. Error "extension cannot be changed" before doing anything
4. try different extensions, fails as well

### Expected behaviour

- allow default extension
- do let user change extension, if it not allow

### Actual behaviour

no way to create file

### Android version

16

### Device brand and model

Samsung S21

### Stock or custom OS?

Stock

### Nextcloud android app version

34.0.0

### Nextcloud server version

Nextcloud Hub 25 Autumn

### Using a reverse proxy?

I don't know

### Android logs

_No response_

### Server error logs

```bash

```

### Additional information

https://github.com/user-attachments/assets/c5df393f-4cd4-4c08-af74-944e81254a78