Describe the bug
As an admin, when trying to import a participatory process, attachments are not imported even if I enable attachments import.

To Reproduce

1. Export a process with attachments and files
2. Open the zip
3. Import the process and enable all imports (see screenshot below)
4. See that the process don't have attachments

Expected behavior
As an admin, when I enable "Import attachments" checkbox, I expect attachments to be imported.

Screenshots

Extra data (please complete the following information):

• Decidim Version: [0.26, 0.27, 0.28.0.dev]
• Decidim installation: [nightly]

Additional context
If it's not possible to add attachments, I think it should be better to delete the "Import attachments" checkbox.
Might be related to #7060