Cover images selected in create dialog aren't stored #1897

Describe the bug

Cover images, that are selected while you create a shelf or a book, do not appear after closing the dialog, i.e. if you visit the shelves / books section, cover images are not shown.

Workaround: Open the edit dialog after you've created the book / shelf and upload the image (again).

Steps To Reproduce

1. Go to 'Shelves'
2. Click on 'New Shelf'
3. Add a title and upload a cover image
4. Click on 'SAVE SHELF'
5. Go to 'Shelves'

Expected behavior

Previously uploaded cover image is shown.

Configuration

• Exact BookStack Version (Found in settings): 0.28.0
• PHP Version: 7.3.11
• Hosting Method (Nginx/Apache/Docker): nginx (+ php-fpm)