MultipartFormDataContent.cs

var uri = new Uri(API_URL_MEDIA);
var content = new MultipartFormDataContent();
content.Add(new StringContent(caption), ""caption"");
var t = new StreamContent(photoContents);
t.Headers.ContentType
    = new System.Net.Http.Headers.MediaTypeHeaderValue(contentType);
fileName = string.IsNullOrEmpty(fileName) ? ""img.png"" : fileName;
content.Add(t, ""media"", """ + fileName + """);
var response = PostBuffer(uri, content);
response.EnsureSuccessStatusCode();