How to post form-data files with C# to an external Api

I have created an API with asp.net core that should accept POST of files and strings, The API works greate with Postman but I'm not sure how to implement the POST action from Postman to my Asp.net Client.

[HttpPost]
    public async Task<IActionResult> PostAttachment([FromForm] Attachment attachment)
    {            
        if (attachment.Files == null)
        {
            return BadRequest("File is required");
        }

        foreach (var file in attachment.Files)
        {
            if (file != null)
            {
                var extension = Path.GetExtension(file.FileName);
                var allowedExtensions = new[] { ".jpg", ".png", ".docx", ".pdf", ".xlsx" };

                if (!allowedExtensions.Contains(extension.ToLower()))
                {
                    return BadRequest("Only the follwing extentions are allowed: .jpg, .png, .docx, .pdf, .xlsx");
                }

                if (file.Length > 5300000)
                {
                    return BadRequest("File size should be less than 5MB");
                }

                var upload = _hostingEnvironment.ContentRootPath + "\\uploads";

                for (int i = 0; i < attachment.Files.Count; i++)
                {
                    var filePath = Path.Combine(upload, file.FileName);
                    using (var fileStream = new FileStream(filePath, FileMode.Create))
                    {
                        await file.CopyToAsync(fileStream);
                        attachment.Path = file.FileName;
                    }
                }
            }
        }

            _context.Attachments.Add(attachment);
            await _context.SaveChangesAsync();

            return Content("Upload Successful.");          

    }

And this is my Attachment model

public class Attachment
{
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int Id { get; set; }
    public string Name { get; set; }
    public string Description { get; set; }

    [Required]
    public string Path { get; set; }
    public DateTime UploadDate { get; set; }

    public Request Request { get; set; }
    public int RequestId { get; set; }

    [NotMappedAttribute]
    public List<IFormFile> Files { get; set; }
}

In Postman I do the following: Postman

and it works great.

I want to know how can i do the same thing that Postman do but on my asp.net Client.

I already tried the code snippet that postman provides with RestSharp but I get the files uploaded with 0 bytes:

var client = new RestClient("https://localhost:44316/api/Attachments");
var request = new RestRequest(Method.POST);
request.AddHeader("cache-control", "no-cache");
request.AddHeader("Connection", "keep-alive");
request.AddHeader("Content-Length", "4097");
request.AddHeader("Accept-Encoding", "gzip, deflate");
request.AddHeader("Content-Type", "multipart/form-data; boundary=--------------------------938323281300032110502154");
request.AddHeader("Host", "localhost:44316");
request.AddHeader("Cache-Control", "no-cache");
request.AddHeader("Accept", "*/*");
request.AddHeader("User-Agent", "PostmanRuntime/7.16.3");
request.AddHeader("content-type", "multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW");
request.AddParameter("multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW", "------WebKitFormBoundary7MA4YWxkTrZu0gW\r\nContent-Disposition: form-data; name=\"Files\"; filename=\"lang.png\"\r\nContent-Type: image/png\r\n\r\n\r\n------WebKitFormBoundary7MA4YWxkTrZu0gW\r\nContent-Disposition: form-data; name=\"Files\"; filename=\"uk.png\"\r\nContent-Type: image/png\r\n\r\n\r\n------WebKitFormBoundary7MA4YWxkTrZu0gW\r\nContent-Disposition: form-data; name=\"Path\"\r\n\r\nstring path\r\n------WebKitFormBoundary7MA4YWxkTrZu0gW\r\nContent-Disposition: form-data; name=\"RequestId\"\r\n\r\n8\r\n------WebKitFormBoundary7MA4YWxkTrZu0gW--", ParameterType.RequestBody);
IRestResponse response = client.Execute(request);

. . UPDATE 1:

I have tried to remove the parameter in the previous code and add this:

request.AddFile("Files", "C:/Users/User/Desktop/First-app/VisitorPortalImgs/Picture2.png", "image/png");
        request.AddFile("Files", "C:/Users/User/Desktop/First-app/VisitorPortalImgs/Picture1.png", "image/png");
        request.AddFile("Files", "C:/Users/User/Desktop/First-app/VisitorPortalImgs/Picture1.png", "image/png");
        request.AddParameter("Path", "Picture1path.png");
        request.AddParameter("UploadDate", "10/21/2019 11:11 AM");
        request.AddParameter("RequestId", "9");
        request.AlwaysMultipartFormData = true;

This time nothing gets uploaded to the api.