JSON:API associated file select the wrong file

Problem/Motivation

Creating an entity (node) in the system using an image for a field will select the wrong image if a file other than an image was inserted in the system before the image.

Steps to reproduce

1. Insert a media type "Document" in Drupal using Content > Media > Add media
2. Go to Content > Files. You will notice 2 new files: the file you just created (FID 1) and a generic.png file (FID 2).
3. Insert an image using Content > Media > Add media
4. Go to Content > Media and check the MID of the image (mouse over the "edit" link). The image MID is 2. Go to Content > Files and check the FID of the image (mouse over the "Used in" link). The image has FID 3. This desync of MID / FID causes a lot of issues with the JSON:API.
5. Connect to the database and retrieve the UUID of the inserted file SELECT * FROM file_managed WHERE fid = 3;
6. Modify the article content type. Add a Reference > Media labelled "Preview" (field_preview).
7. Insert a node using the JSON:API, with the UUID of the inserted image:

{
  'data': {
    'type': 'node--article',
    'attributes': {
      'title': 'Article title',
      'status': 1
    },
    'relationships': {
      'field_preview': {
        'data': {
          'type': 'file--file',
          'id': '30997b47-d065-4e52-980e-28a7b8ee5e9e'
        }
      }
    }
  }
}

Instead of using the image with the UUID specified, Drupal resolve the MID of the specified image (which is 2), find the file with the FID that matched the MID of the image (FID 2 is the generic.png file that was automatically inserted by Drupal) and associate the generic.png image as preview image to the article.

The JSON:API confuses MID and FID.

Proposed resolution

Make sure the JSON:API does not assume MID is the same as FID.

Remaining tasks

Find where in the code the MID / FID confusion happens, and fix it.

Release notes snippet

(will insert more info later if needed)