Multistep File Upload not persisting visually when navigating steps + existing uploaded file not rendered in edit mode

[alexanderloewe]

Describe the bug

We are experiencing a critical issue with file uploads in Formie when used inside multistep forms and when editing existing submissions.

The file upload itself works correctly: the file is uploaded and stored in the submission in the backend. However, the frontend behaviour is inconsistent and leads to confusion for users.

1. Multistep Form – File disappears visually when navigating steps

Steps to reproduce

1.	Create a multistep form.
2.	Add a File Upload field.
3.	Upload a file in step 1.
4.	Navigate to the next step.
5.	Navigate back to the previous step.

Expected behaviour

•	The previously uploaded file should still be visible or indicated as uploaded.
•	The field should reflect the stored submission state.

Actual behaviour

•	The file appears to be missing in the frontend UI.
•	In Craft backend, the file is correctly stored in the submission.
•	This creates uncertainty for users, who assume the upload failed.

2. Submission Editing – Existing File not rendered in frontend

We are also using Formie submissions as an editable waiting list system.

Expected behaviour

•	Previously uploaded file should be shown or referenced.
•	User should clearly see which file is already attached.

Actual behaviour

•	File exists in backend submission.
•	File Upload field renders empty in frontend.
•	User cannot verify or associate the existing file with their submission.

Steps to reproduce

Form settings

• Multi-page form: Yes
• Submission Method: Ajax
• Client-side Validation: Yes
• Custom Form Templates: No

Craft CMS version

5.9.13

Plugin version

3.1.10

Multi-site?

NO

Additional context

No response

[engram-design]

For ajax-based forms, it's a known issue at the moment. HTML file input fields are read-only, so we can't populate them after upload to show it has a value, unlike simple text-based input fields. As such, we have to have a written note about what files have been uploaded, which are now assets. Users can then upload more to replace what was uploaded. This basic functionality works fine for Page Reload forms, but Ajax-based forms would need a mechanism for getting that HTML back from the server, and changing its field content client-side. We just haven't built the infrastructure for that yet, because no other field needs that until now.

We've been putting it off, mostly in favour of dedicating work to an entirely better file upload solution.