title: Can't convert HttpStringContent to HttpContent
body: <p>I try create Windows Store app. And i need send post request, but i have a problem. 
  My method:</p>
  
  <pre><code>       private async Task&lt;string&gt; POST(byte[] pic, string upload_url)
      {
  
  
          var multipartContent = new System.Net.Http.MultipartFormDataContent();
          multipartContent.Add(new System.Net.Http.ByteArrayContent(pic), "photo");
          var client = new HttpClient();
  
          var response = await client.PostAsync(
  new Uri(upload_url),
  multipartContent);
          return await response.Content.ReadAsStringAsync();
      }
  </code></pre>
  
  <p>Always i have Task member after return. With status-Waiting for activation. Where error? Thank you! </p>