ASP.NET Core Web API ApiController request is null for POST but files parameter is available

In the code shown here, the PostFormData method receives the files parameter as it should but the Request object is null and I don't know why.

public class UploadController : ApiController
{
    private readonly IHttpContextAccessor _httpContextAccessor;
    private readonly IWebHostEnvironment _webHostEnvironment;

    public UploadController(IHttpContextAccessor httpContextAccessor,
                            IWebHostEnvironment webHostEnvironment)
    {
        _httpContextAccessor = httpContextAccessor;
        _webHostEnvironment = webHostEnvironment;
    }

    [HttpPost]
    public async Task<IHttpActionResult> PostFormData(List<IFormFile> files, CancellationToken cancellationToken)
    {
        try 
        {
            // Check if the request contains multipart/form-data.
            if (!Request.Content.IsMimeMultipartContent())
            {
                throw new HttpResponseException(HttpStatusCode.UnsupportedMediaType);
            }
        }
    }
}

Network headers

:authority
localhost:7196
:method
POST
:path
/upload/PostFormData
:scheme
https
accept
*/*
accept-encoding
gzip, deflate, br, zstd
accept-language
en-GB,en;q=0.9,en-US;q=0.8,en-AU;q=0.7
cache-control
max-age=0
content-length
1293
content-type
multipart/form-data; boundary=----WebKitFormBoundaryy5g1PB4Exs6s0PYA
dnt
1
origin
null
priority
u=1, i
sec-ch-ua
"Not;A=Brand";v="99", "Microsoft Edge";v="139", "Chromium";v="139"
sec-ch-ua-mobile
?0
sec-ch-ua-platform
"Windows"
sec-fetch-dest
empty
sec-fetch-mode
cors
sec-fetch-site
cross-site
user-agent
Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/139.0.0.0 Safari/537.36 Edg/139.0.0.0