uploading png with wwwform

I'm trying to upload a png to a service but I'm getting the following www error:

necessary data rewind wasn't possible.

I'm working on a mac standalone with unity 4.6.

I looked up the similar errors people posted about redirects, or problems with php on the their own servers, but this is a known service that accepts thousands of photo uploads a day so it is not those problems. The instructions they gave were the following:

 <form action="http://www.someservice.com/upload"
       method="post"
       enctype="multipart/form-data">
     <input type="text"     name="username" value="">
     <input type="password" name="password" value="">
     <input type="yourfile"     name="file">
 </form>

This is what I'm trying to duplicate in Unity.

The code I'm using to grab and save the picture is below. Grabbing and saving the picture work fine. I can open it in my streaming assets folder after the code runs:

 var tex = new Texture2D (picwidth, picheight, TextureFormat.RGB24, false);
         // Read screen contents into the texture Rect(0,0 is lower left and then goes up the width and height I think.
         tex.ReadPixels (Rect(left, screenheight-top-picheight, picwidth, picheight), 0, 0);
         tex.Apply ();
 
         // Encode texture into PNG
         var bytes = tex.EncodeToPNG();
         File.WriteAllBytes(Application.streamingAssetsPath + "/pic.png", bytes);

 var form = new WWWForm();
 
         form.AddBinaryData("file",bytes,"pic.png","image/png");
         form.AddField("username","myusername");
         form.AddField("password","mypassword");
 
         // Upload to a cgi script
         var w = WWW("http://www.someservice.com/upload", form);
         yield w;
         if (w.error != null) {
             print(w.error);
         } else {
         Debug.Log(w.text);
             print("Finished Uploading Screenshot");
         }