How to send multipart/form-data to ASP.NET Core Web API?

Maybe you should try decorate controller input and model with [FromForm] attribute? See more info here: web api parameters binding.

In given example your controller action should look like this:

[HttpPost("/api/account"), Authorize]
public void SaveUser([FromForm]UserModel info)

In model:

[FromForm(Name="avatar")]
public IFormFile Avatar { get; set; }

[FromForm(Name="name")]
public string Name { get; set; }

1-send multi part file

            [HttpPost, Route("postimagenews")]
            public IActionResult PostImageNews([FromForm] IFormFile file)
            {
                try
                {
                    if (file == null || file.Length == 0)
                    {
                        return BadRequest("Please send a photo");
                    }
                    //create unique name for file
                    var fileName = Guid.NewGuid().ToString() + Path.GetExtension(file.FileName);
    
                    //set file url
                    var savePath = Path.Combine(Directory.GetCurrentDirectory(), "wwwroot/images/news", fileName);
    
                    using (var stream = new FileStream(savePath, FileMode.Create))
                    {
                        file.CopyTo(stream);
                    }
    
                    return Ok(fileName);
                }
                catch
                {
                    return BadRequest("error in upload image");
                }
            }

2-recive multi part file

            public static async Task<string> PostImage(string apiendpoint, IFormFile data)
            {
                using (var httpClient = new HttpClient())
                {
                    var multipartContent = new MultipartFormDataContent();
                    var fileContent = new ByteArrayContent(GetFileArray(data));
                    fileContent.Headers.ContentType = MediaTypeHeaderValue.Parse("image/jpeg");
                    multipartContent.Add(fileContent, "file",data.FileName);
                    var resultUploadImage= await httpClient.PostAsync(apiendpoint, multipartContent);
                    if (resultUploadImage.IsSuccessStatusCode)
                    {
                        var fileName=(await resultUploadImage.Content.ReadAsStringAsync()).Replace("\"", "");
                        return fileName;
                    }
                    return "";
                }
            }
    
            public static byte[] GetFileArray(IFormFile file)
            {
                using (var ms = new MemoryStream())
                {
                    file.CopyTo(ms);
                    return ms.ToArray();
                }
            }

3-send file multi part with postman