When uploading a new file, properties fail to be cleared/removed

See attached video.

1. Edit an existing record on a collection that supports file attachments
2. Clear out a property
3. Attach a new file
4. Click update
5. Note that the property you tried to clear has the old value in it

If you change the property to something else, it will change as expected. Only trying to set it to blank/undefined causes this issue.

[video](https://github.com/Kinto/kinto-admin/assets/148472676/a890fada-b2b3-443b-a442-c2517700778b)

Reproduce the issue in the Kinto Web Administration Console using the five steps in the report, focusing on an existing record with file attachments. Trace the update flow for cleared properties when a new file is attached. Done means a blank or undefined property stays cleared after the record is updated, while changing it to another value continues to work.

Written by the indexing model from the issue text.