Can't delete an attachment to my proposal as a user

[paulinebessoles]

Describe the bug
As a user creating a proposal, after I attached a file or an image to my proposal, I can't delete it, neither by returning to step 4 of the creation wizard, nor afterwards when modifying the proposal.

To Reproduce

1. Go to 'https://try.decidim.org/processes/budget-participatif-2021/f/8/'
2. Create a proposal and attach an image to it
3. Preview the proposal
4. Go back to step 4, delete the image
5. Preview again
6. See error

Expected behavior
As a user, I want to be able to delete attachments on my proposals.

Extra data (please complete the following information):

• Device: [Desktop]
• Device OS: [MacOs Big Sur]
• Browser: [Firefox]
• Decidim Version: [0.23.1 / 0.23.2]
• Decidim installation: [try.decidim.org]

[ahukkanen]

I think this is related to #7452 (original issue #6880).

[andreslucena]

I think this is related to #7452 (original issue #6880).

I can confirm this is indeed the same error as #7452 and that it's already solved. I've just checked it in develop/v0.26.0.dev