Magento 2 Dynamically Add file uploader in system config

This does not fully answer my question but partially and puts it in the right direction:

I added now file uploader look like so:

Now when I save serialised data looks like this:

{
  "_1565251449037_37": {
    "category_id": "5",
    "file_uploaded": {
      "name": "",
      "type": "",
      "tmp_name": "",
      "error": 4,
      "size": 0
    }
  },
  "_1565251729340_340": {
    "category_id": "7",
    "file_uploaded": {
      "name": "wed2b_regions_stores_v1.csv",
      "type": "text\/csv",
      "tmp_name": "\/private\/var\/tmp\/phpZwzFAH",
      "error": 0,
      "size": 10591
    }
  }
}

When it return to the page refreshed I am getting the error:

I understand why the error is because the element is a file element button and it can't be set plus I think magento is expecting the "file_uploaded" to be string and is an object and not a string when perhaps value should be file_uploaded.name

Any help on this appreciated.