"Make a Copy" does not copy attached files

On alpha.iodide.io revision 1d87c1a1c8, when the user clicks on "Make a Copy", the notebook is cloned, but none of the files attached to it, which may prevent the notebook from working.

### What I Did

1. Create a new notebook
2. Go to Files > Add a file, say "test.png"
3. Verify that the file is accessible under /notebooks//files/test.png
4. Click on "Make a Copy"
5. With the new notebook, check if the file is accessible unter /notebooks//files/test.png

### What I Expected

The file should display.

### What Happened

500 Internal Server Error message

### (if applicable) notebook that reproduces bug

Original notebook: https://alpha.iodide.io/notebooks/4281/files/kaefer.png
Copy: https://alpha.iodide.io/notebooks/4283/files/kaefer.png

Reproduce the issue on alpha.iodide.io using the notebook and file-copy steps in the report, then trace the “Make a Copy” flow and attached-file handling. Done means the copied notebook can access the attached file at its new /notebooks//files/ path without a 500 error.

Written by the indexing model from the issue text.