<input type="file" id="script_to_upload"name="files">

[HttpPost("UploadFiles")]
public async Task<IActionResult> Post(List<IFormFile> files)
{
    long size = files.Sum(f => f.Length);

    // full path to file in temp location
    var filePath = Path.GetTempFileName();

    foreach (var formFile in files)
    {
        if (formFile.Length > 0)
        {
            using (var stream = new FileStream(filePath, FileMode.Create))
            {
                await formFile.CopyToAsync(stream);
            }
        }
    }

    return Ok(new { count = files.Count, size, filePath});
}

function DoUpload() {
                if (window.FormData !== undefined) {
                    var fileUpload = $("#script_to_upload").get(0);
                    var files = fileUpload.files;

                    var fileData = new FormData();

                    // Looping over all files and add it to FormData object  
                    for (var i = 0; i < files.length; i++) {
                        fileData.append(files[i].name, files[i]);
                    }

                    fileData.append('parent_id', 'Vasya');
                    fileData.append('username', 'Zelya');

                    $.ajax({
                        url: '/Db/UploadScript',
                        type: "POST",
                        contentType: false, // Not to set any content header  
                        processData: false, // Not to process data  
                        data: fileData,
                        dataType: 'json',
                        success: function (result) {
                            alert(result);
                        },
                        error: function (err) {
                            alert(err.statusText);
                        }
                    });
                } else {
                    alert("FormData is not supported.");
                }  
            }

 [HttpPost]
        public ActionResult UploadScript()
        {
            string Name = "Kuku";
            try
            {
                var files = HttpContext.Request.Form.Files; //<--- улетает

                Name = Request.Form["username"];//<---улетает
            }
            catch(Exception x)
            {
                Console.WriteLine(x.Message);
            }

            return Content("OK");
        }