Describe the bug
There is an error occuring when an Admin is trying to add an optional Attachment into a Proposal even though the Proposal component is configure to allow attachment.
Normal users, if they are allowed to, can create proposal with optional Attachment if Proposal Component are set up to allowed optional Attachment.

To Reproduce
Steps to reproduce the behavior:

1. In a Participatory Process add a Proposal Component and make sure to check the box Allow attachments
2. As an Admin create a new Proposal
3. Fill the Title, Body and add an Optional File and fill the name for this file
4. Click on 'Create'
5. See error

Expected behavior
The Proposal should be created with the optional File because the Proposal Component was set up to Allow attachments

Screenshots
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Proposal Component with Allow attachments checked

Attaching a file to the new proposal and click create

The error on try.decidim.org

See more verbose error

The function thatlaunches the error

Stacktrace
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Extra data (please complete the following information):

• Device: Desktop
• Device OS: Windows 10
• Browser: Chrome
• Decidim Version: 0.27.1 and try.decidim.org

Additional context
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