Interpreting base 64 in C # from an image based on JSON / PHP (base64_encode)

So, I can successfully read the image file and transfer it back to my C # application, but I cannot decode it correctly.

I am returning JSON data as such (the json_encode function is not shown) via PHP:

        $imgbinary = fread(fopen($filename, "r"), filesize($filename));

        if ( strlen($imgbinary) > 0 ){
            return array("success"=>true, "map"=>base64_encode($imgbinary));
        }

Then in C # I use Newtonsoft.Json to decode the string (I can successfully read the success and properties of the map), but I cannot use base64 decoding to write the image to the file correctly (or to display).

I am doing this as such:

File.WriteAllText(System.Windows.Forms.Application.StartupPath + "\\MyDir\\" + FileName, Base64Decode(FileData));

    public string Base64Decode(string data)
    {
        byte[] binary = Convert.FromBase64String(data);
        return Encoding.Default.GetString(binary);
    }

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