async Task<string> DoCamCardAsync()
    {
        var cli = new HttpClient();
        cli.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

        // Why use a custom boundary for multipart/form-data?
        // .NET sticks the thing in quotes in the header; the CamCard server goes nuts for the quotes (takes them literally); while the
        // actual boundary is delineated in the request by the unquoted boundary.
        var boundary = Guid.NewGuid().ToString();
        using (var form = new MultipartFormDataContent(boundary))
        {
            form.Headers.Remove("Content-Type");
            form.Headers.TryAddWithoutValidation("Content-Type", "multipart/form-data; boundary=" + boundary);

            byte[] cardBytes = File.ReadAllBytes(@"file path here, or use stream instead");
            var fileContent = new ByteArrayContent(cardBytes);
            fileContent.Headers.ContentDisposition = new ContentDispositionHeaderValue("form-data")
            {
                Name=""upfile"",
                FileName = ""card_m.jpg""
            };
            fileContent.Headers.ContentType = new MediaTypeWithQualityHeaderValue("image/jpeg");
            form.Add(fileContent);

            var sURl = "https://bcr1.intsig.net/BCRService/BCR_VCF2?user=[your username here]&pass=[your key here]&json=1&lang=1";
            var response = await cli.PostAsync(sURl, form);
            return await response.Content.ReadAsStringAsync();
        }