MultipartUpload.cs

public static T HttpUploadFile<T>(string url, string fileName, byte[] fileBytes, string fileContentType = null, string fileFieldName = "file", Dictionary<string, string> extraFields = null)
            where T : class
        {

            var client = new HttpClient();
            var request = new HttpRequestMessage(HttpMethod.Post, new Uri(url));

            var boundary = "Upload----" + DateTime.UtcNow.Ticks.ToString("x");
            var multiPartContent = new MultipartFormDataContent(boundary);

            //Add parameters
            if(extraFields != null) { 
                foreach (var field in extraFields)
                {
                    multiPartContent.Add(new StringContent(field.Value), '"' + field.Key + '"'); //Extra quotes are needed for stripe API
                }
            }

            //Add file upload
            var byteContent = new ByteArrayContent(fileBytes);
            byteContent.Headers.ContentDisposition = new ContentDispositionHeaderValue("form-data")
            {
                FileName = '"' + fileName + '"',
                Name = '"' + fileFieldName + '"' //Extra quotes are needed for stripe API
            };

            if(!string.IsNullOrEmpty(fileContentType))
               byteContent.Headers.ContentType = new MediaTypeHeaderValue(fileContentType);

            multiPartContent.Add(byteContent);


            //Set content
            request.Content = multiPartContent;

            //Add Authorization Header
            request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", StripeSecretKey);


            //
            // Handle response
            //

            var response = client.SendAsync(request).Result;
            var responseFromServer = response.Content.ReadAsStringAsync().Result;

            if (response.IsSuccessStatusCode)
                return JsonConvert.DeserializeObject<T>(responseFromServer);

            throw CustomStripeApi.BuildStripeException(response.StatusCode, responseFromServer);
            

        }
        
        public static DocumentUpload UploadIdentityPhoto(IdentityPhoto idPhoto)
        {
            var fields = new Dictionary<string, string>() { { "purpose", "identity_document" } };

            try
            {
                return HttpUploadFile<DocumentUpload>(
                    url: "https://uploads.stripe.com/v1/files", 
                    fileName: idPhoto.FileNameWithExtension, 
                    fileBytes: idPhoto.Content,
                    fileContentType: idPhoto.ContentType, 
                    fileFieldName: "file", extraFields: fields);
            }
            catch(StripeException ex){
             //Log error
            }
        }