Attachment

public void postAttachment(string[] files, string <objectID>, string token, string companyId)
{
    using (WebClient client = new WebClient())
    {
        client.Headers.Add("ipp-company-id", "<companyId>");
        client.Headers.Add("ipp-application-type", "Visma.net Financials");
        client.Headers.Add("Authorization", "bearer<token>");
        
        for (int i = 0; i < files.Length; i++) // Multiple
        {
            byte[] responseArray = client.UploadFile(
                @"https://api.finance.visma.net/v1/<Endpoint>/<objectID>/Attachment", 
                "POST", 
                files[i]
            );
        }
    }
}

 public void postAttachmentHttpClient()
        {
                using (var client = new HttpClient())
                {
 
                    client.DefaultRequestHeaders.Add("ipp-application-type", "Visma.net Financials");
                    client.DefaultRequestHeaders.Add("ipp-company-id", "<companyID>");
                    client.DefaultRequestHeaders.Add("Authorization", "Bearer <token>");

                    HttpResponseMessage response;
                    var content = new MultipartFormDataContent();

                    var path = Path.Combine(@"<path>");
                    string fileName = Path.GetFileName(path);

                    FileStream fs = File.OpenRead(path);
                    var sc = new StreamContent(fs);

                    sc.Headers.Add("Content-Type", "application/octet-stream");
                    sc.Headers.Add("Content-Disposition","form-data; name=\"file\"; filename=\"" + Path.GetFileName(path) + "\"");
                    
                    content.Add(sc, "file", fileName);
                    response = client.PostAsync(@"https://api.finance.visma.net/v1/<endpoint>/<objectID>/Attachment", content).Result;
                }
            }