Transferring a group does not move its projects' or subgroups' projects' upload files, breaking them

Summary

Transferring a group does not move its projects' or subgroups' projects' upload files, breaking them.

Steps to reproduce

1. Create project bar in root group foo
2. Create issue in project and add an attachment
3. Create group baz
4. Transfer group foo to group baz

Visit the issue and see the attachment 404.

What is the expected correct behavior?

Visit the issue and see the attachment load.

Output of checks

This bug happens on GitLab.com.