This is the correct way to upload a file, however you aren't getting the uploadId which would be required to get the file back from server. To do so you need to subscribe to the UploadCompleteMessage and get the uploadId from the UploadCompleteMessage that comes after a successful upload.

Here's a sample script that takes a screenshot, uploads it and can download it:

using UnityEngine;
using System.Collections;
using GameSparks.Api.Requests;

using GameSparks.Api.Messages;

public class UploadFile : MonoBehaviour {

	private string lastUploadId;
	public Texture2D downloadedImage;

	public void Start()
	{
		//We will be passing all our messages to a listener function
		UploadCompleteMessage.Listener += GetUploadMessage;
	}

	//Take a get our upload url
	public void UploadScreenShot () {
		new GetUploadUrlRequest().Send((response) =>
		{
			//Start coroutine and pass in the upload url
			StartCoroutine(UploadAFile(response.Url));	
		});
	}

	//Our coroutine takes the upload url
	public IEnumerator UploadAFile(string uploadUrl)
	{

		yield return new WaitForEndOfFrame();

		int width = Screen.width;
		int height = Screen.height;
		Texture2D tex = new Texture2D(width, height, TextureFormat.RGB24, false);
		tex.ReadPixels(new Rect(0, 0, width, height), 0, 0);
		tex.Apply();
		//This basically takes a screenshot

		byte[] bytes = tex.EncodeToPNG(); //Can also encode to jpg, just make sure to change the file extensions down below
		Destroy(tex);

		// Create a Web Form
		var form = new WWWForm();
		form.AddField("somefield", "somedata");
		form.AddBinaryData("file", bytes, "hehe.png", "image/png");

		WWW w = new WWW(uploadUrl, form);
		yield return w;

		if (w.error != null)
		{
			Debug.Log(w.error);
		}
		else
		{
			Debug.Log(w.text);
		}
	}

	//This will be our message listener
	public void GetUploadMessage(GSMessage message)
	{
		//Every time we get a message
		Debug.Log(message.BaseData.GetString("uploadId"));
		//Save the last uploadId
		lastUploadId = message.BaseData.GetString("uploadId");
	}

	//When we want to download our uploaded image
	public void DownloadAFile()
	{
		//Get the url associated with the uploadId
		new GetUploadedRequest().SetUploadId(lastUploadId).Send((response) =>
			{
				//pass the url to our coroutine that will accept the data
				StartCoroutine(DownloadImage(response.Url));
			});
	}

	
	public IEnumerator DownloadImage(string downloadUrl)
	{
		var www = new WWW(downloadUrl);

		yield return www;

		downloadedImage = new Texture2D(200, 200);

		www.LoadImageIntoTexture(downloadedImage);
	}
}

You can save uploadIds to a leaderboard entry, challenge or user using scriptData.

Shane