m4b: Book cover not taken from file

### Overview

When adding an m4b file I expect the cover to taken from the file. However, the app does not display any cover image for the book.

### How to reproduce

1. Add a m4b file
2. look at book list
3. See how no cover image exists

### Version

25.12.1 F-Droid

### Android Version

16

### Device Model

Pixel 7