Bug with adding custom media_folder. Directory not added to file location.

[jeremiahfallin]

Describe the bug
When using a file widget with a separate media folder, the correct directory pops up when choosing a file, but after selecting one the directory is not added to the path in the md file. Just the file name.

Tried adding it like this:

      - file: "src/pages/board/index.md"
        label: "Board Page"
        name: "board"
        media_folder: '/static/board'

and also to the widget itself:

{ label: File, name: file, media_folder: '/static/board', widget: file },

and neither of those seem to work.

To Reproduce

1. Add a separate media folder to a file based page.
2. Add a file widget.
3. Try to add a file from the CMS to the web page.
4. See that the directory is not added to the file's stored path.

Expected behavior
The path to the file should also be included like it is when you're not using a separate media folder.

Applicable Versions:

• Netlify CMS version: 2.11.32
• Git provider: GitHub

CMS configuration
I included the relevant info at the top.

Additional context
Related to feat #3208. Adding a public_folder field prepends the directory, like so:

      - file: "src/pages/board/index.md"
        label: "Board Page"
        name: "board"
        media_folder: '/static/board'
        public_folder: '/static/board'