Describe the bug
When editing a proposal with an image and adding an attachment, the image disappears.

To Reproduce
Steps to reproduce the behavior:

1. Create a proposal with an image.
2. Edit the proposal and add an attachment.
3. See error (image has disappeared).

Expected behavior
I'd expect that when updating an attachment the image will not disappear.

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