File widget ignores its own media_folder and instead uses media_folder from the root

[cshaa]

Describe the bug
My team is working on a static web page that has a lot of images that are preprocessed by Vite. That's why we have media_folder: "src/assets/uploads" in the root of the config (that's where images are supposed to go). Now, we want to add the option to upload files, which should be placed directly in the public folder, so we used the media_library.media_folder property on the file widget – but it is ignored, and the files are uploaded to src/assets/uploads instead of public/uploads.

To Reproduce

1. Create the following config.yml:

local_backend: false
backend:
  name: gitlab
  repo: ...
  branch: main
  auth_type: pkce
  api_root: https://gitlab.druit.org/api/v4
  base_url: https://gitlab.druit.org
  auth_endpoint: oauth/authorize
  app_id: ...
locale: cs
publish_mode: simple
media_folder: src/assets/uploads
public_folder: "#uploads"
collections:
  - ...
  - name: files
    label: Files
    folder: src/content/files
    create: true
    fields:
      - widget: file
        name: file
        media_library:
          allow_multiple: false
          media_folder: public/uploads

2. Go to administration > Files and uplad a file
3. See that it was uploaded to src/assets/uploads and not public/uploads

Applicable Versions:

• Decap CMS version: 3.4.0
• Git provider: GitLab

• Node.JS version: 23.3

[FelixBrakel]

You can work around this by defining the media_folder and public_folder one level higher in the config. That is on the field instead of the media_library. This does come with its own strange behavior though. Consider the following example

media_folder: "public/upload"
public_folder: "/upload"
collections:
  - name: "some collection"
  - ...
    fields:
        - { label: "Title", name: "title", widget: "string" }
        - label: "Cover Image"
          name: "image"
          widget: "image"
          choose_url: false
          media_library:
            allow_multiple: false
        - label: "Articles"
          name: "articles"
          widget: "list"
          fields:
            - { label: "Title", name: "title", widget: "string" }
            - label: "PDF"
              name: "pdf"
              widget: "file"
              choose_url: false
              media_folder: "public/archive"
              public_folder: "/archive"
              media_library:
                allow_multiple: false

This will correctly store the PDFs in the public/archive folder and serve them at /archive. It will also correctly store any images at public/upload and serve them at /upload. However the file picker of the Cover Image will look for files at public/archive.

A workaround for this on top of the first workaround is to redefine the media_folder and public_folder as /public/upload and /upload on the Cover Image field.