I've come across a scenario in which I’m now unable to delete files using the user interface.

Steps to replicate

1. Create a file resource within a course
2. Add an image to the file resource and save changes
3. Within the same course, add a page resource
4. Within the page resource, add an image (Browser Repositories > Server files > The image you uploaded previously, click 'Make a copy of the file')
5. Save changes to the page
6. Within the course, now delete File resource originally uploaded
7. Go to the Page resource and check the image still loads
8. Try and delete the image uploaded in the server files area using the file picker.