The mod_assign activity instructions don't retain images when the course is backed up.

Steps to reproduce:

1. Create an assignment activity (doesn't need anything special).
2. Add some text and and image to the 'Activity instructions' text area (I'm using Tinymce).
3. Save.
4. Backup the course and load into another Moodle instance (can't be the same one).
5. The image in the 'activity instructions' is broken.

You can confirm this by extracting the MBZ and looking at the files.xml. The file is not included in the backup.