JsonMultipartFormDataSupport

Morasiu's project is taken as a basis.
The main difference from the original source is reading the json body from Request.Form.Files.

Usage

1. Simple add this to your ConfigureServices

services.AddJsonMultipartFormDataSupport(JsonSerializerChoice.SystemText);

2. Create your wrapper

public class MyWrapper
{
    [Required]
    public IFormFile File { get; set; } = null!;

    [FromJson, Required]
    public MyJson Json { get; set; } = null!;
}

and then add to your controller

[HttpPost]
public IActionResult Post([FromForm] MyWrapper request)
{
    return this.Ok();
}