How do I create a multipartformdata in for this specific querystring?

I was trying to form a MultipartFormDataContent. With a valid request, it should look like below:

Content-Disposition: form-data; name=""; filename="rst.png"
Content-Type: image/png

But I am getting something like below:

Content-Disposition: form-data; name=rst.png; filename=rst.png; 

And this is what I have tried

using (var httpClient = new HttpClient())
    using (var newRequest = new HttpRequestMessage(HttpMethod.Post, MYAPIURI))
        newRequest.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
        MultipartFormDataContent multipartContent = new MultipartFormDataContent();
        var jsonContent = new StringContent(JsonConvert.SerializeObject(preflightRequestSetting), Encoding.UTF8, "multipart/form-data");
        multipartContent.Add(jsonContent, "Key");                        

        var filename = "rst.png;"
        //multipartContent.Add(new ByteArrayContent(pdfBytes), "DOES NOT MATTER", "MATTERS!");
        multipartContent.Add(new ByteArrayContent(MY_BYTE_ARRAY), fileName, fileName);

        using (var response = await httpClient.PostAsync(MYAPIURI, multipartContent).ConfigureAwait(false))
            if (response.IsSuccessStatusCode)
                var json = response.Content.ReadAsStringAsync().Result;
                var responseStructure = JsonConvert.DeserializeObject<MyResponseClass>(json);
                return responseStructure;

Can anyone please help me finding out the reason what am I doing wrong here? Why I am sending two filenames instead of content type? I appreciate your help.