Describe the bug

When editing a proposal/initiative the attachments do not update correctly.

• If you add an attachment, it replaces the previous ones.
• Deletion of attachments doesn't work.

To Reproduce
Steps to reproduce the behavior:

1. Create a proposal or initiative with more than one attachment.
2. Edit the proposal or initiative.
3. Delete one of the attachments and save.
4. See error (attachment has not been deleted).
5. Add a new attachment
6. See error (the old attachments have been replaced by the new one).

Expected behavior
I would expect:

• Be able to delete an attachment.
• Add an attachment to existing attachments, not replace them.

Screenshots

Stacktrace
If applicable, add the error stacktrace to help explain your problem.

Extra data (please complete the following information):

• Device: Desktop
• Device OS: Ubuntu 20.04
• Browser: Firefox
• Decidim Version: 0.24.0.rc2
• Decidim installation: MetaDecidim