Post data with files using ASP.NET Core MVC to api (ASP.NET Core Web API)

I need to post data with files but I face this problem - all data is null.

It works when I use postman:

My post function in ASP.NET Core Web API:

    public async Task<ActionResult<Category>> PostCategory([FromForm]CategoryViewModel model)
    {
        Category category = new Category()
        {
            Brief = model.Brief,
            Color = model.Color,
            IsDraft = model.IsDraft,
            Name = model.Name,
            Priority = model.Priority,
            Update = DateTime.Now
        };

        if (model.Files != null)
        {
            category.IconUrl = ApplicationManager.UploadFiles(model.Files, "content/category")[0];
        }

        _context.Categories.Add(category);
        await _context.SaveChangesAsync();

        return CreatedAtAction("GetCategory", new { id = category.Id }, category);
    }

My post function in ASP.NET Core MVC:

    private ApiClient _client;
    _client = new ApiClient(new Uri("https:localhost:55436/api/"));

    public async Task<IActionResult> Create([FromForm]CategoryViewModel category)
    {
        var uri = new Uri(_appSettings.WebApiBaseUrl + "Categories");
        var response = await _client.PostAsync<Category, CategoryViewModel>(uri, category);
        return RedirectToAction("index");
    }

My ApiClient class:

public partial class ApiClient
{
    private readonly HttpClient _httpClient;
    private Uri BaseEndpoint { get; set; }

    public ApiClient(Uri baseEndpoint)
    {
        if (baseEndpoint == null)
        {
            throw new ArgumentNullException("baseEndpoint");
        }

        BaseEndpoint = baseEndpoint;
        _httpClient = new HttpClient();
    }

    /// <summary>  
    /// Common method for making GET calls  
    /// </summary>  
    public async Task<T> GetAsync<T>(Uri requestUrl)
    {
        addHeaders();
        var response = await _httpClient.GetAsync(requestUrl, HttpCompletionOption.ResponseHeadersRead);
        response.EnsureSuccessStatusCode();
        var data = await response.Content.ReadAsStringAsync();
        return JsonConvert.DeserializeObject<T>(data);
    }

    public async Task<HttpResponseMessage> PostStreamAsync(Uri requestUrl, object content)
    {
        using (var client = new HttpClient())
        using (var request = new HttpRequestMessage(HttpMethod.Post, requestUrl))
        using (var httpContent = CreateHttpContentForStream(content))
        {
            request.Content = httpContent;

            using (var response = await client
                .SendAsync(request, HttpCompletionOption.ResponseHeadersRead)
                .ConfigureAwait(false))
            {
                response.EnsureSuccessStatusCode();
                return response;
            }
        }
    }

    public async Task<HttpResponseMessage> PostBasicAsync(Uri requestUrl, object content)
    {
        using (var client = new HttpClient())
        using (var request = new HttpRequestMessage(HttpMethod.Post, requestUrl))
        {
            var json = JsonConvert.SerializeObject(content);

            using (var stringContent = new StringContent(json, Encoding.UTF8, "application/json"))
            {
                request.Content = stringContent;

                using (var response = await client.SendAsync(request, HttpCompletionOption.ResponseHeadersRead)
                                                  .ConfigureAwait(false))
                {
                    response.EnsureSuccessStatusCode();
                    return response;
                }
            }
        }
    }

    public static void SerializeJsonIntoStream(object value, Stream stream)
    {
        using (var sw = new StreamWriter(stream, new UTF8Encoding(false), 1024, true))
        using (var jtw = new JsonTextWriter(sw) { Formatting = Formatting.None })
        {
            var js = new JsonSerializer();
            js.Serialize(jtw, value);
            jtw.Flush();
        }
    }

    private static HttpContent CreateHttpContentForStream<T>(T content)
    {
        HttpContent httpContent = null;

        if (content != null)
        {
            var ms = new MemoryStream();
            SerializeJsonIntoStream(content, ms);
            ms.Seek(0, SeekOrigin.Begin);
            httpContent = new StreamContent(ms);
            httpContent.Headers.ContentType = new MediaTypeHeaderValue("application/json");
        }

        return httpContent;
    }

    /// <summary>  
    /// Common method for making POST calls  
    /// </summary>  
    public async Task<T> PostAsync<T>(Uri requestUrl, T content)
    {
        addHeaders();
        var response = await _httpClient.PostAsync(requestUrl.ToString(), CreateHttpContent<T>(content));

        response.EnsureSuccessStatusCode();
        var data = await response.Content.ReadAsStringAsync();
        return JsonConvert.DeserializeObject<T>(data);
    }
    public async Task<T1> PostAsync<T1, T2>(Uri requestUrl, T2 content)
    {
        addHeaders();
        var response = await _httpClient.PostAsync(requestUrl.ToString(), CreateHttpContent<T2>(content));
        response.EnsureSuccessStatusCode();

        var data = await response.Content.ReadAsStringAsync();
        return JsonConvert.DeserializeObject<T1>(data);
    }

    public Uri CreateRequestUri(string relativePath, string queryString = "")
    {
        var endpoint = new Uri(BaseEndpoint, relativePath);
        var uriBuilder = new UriBuilder(endpoint);
        uriBuilder.Query = queryString;
        return uriBuilder.Uri;
    }

    public HttpContent CreateHttpContent<T>(T content)
    {
        var json = JsonConvert.SerializeObject(content, MicrosoftDateFormatSettings);
        var value = new StringContent(json, Encoding.UTF8, "application/json");
        return value;
    }

    public static JsonSerializerSettings MicrosoftDateFormatSettings
    {
        get
        {
            return new JsonSerializerSettings
            {
                DateFormatHandling = DateFormatHandling.MicrosoftDateFormat
            };
        }
    }

    public void addHeaders()
    {
        _httpClient.DefaultRequestHeaders.Remove("userIP");
        _httpClient.DefaultRequestHeaders.Add("userIP", "192.168.1.1");
    }
}