problem with curl piping $_POST and $_FILES in same request

Question

I'm trying to recreate an entire http request including both post and files data however no matter what I do I can't seem to get my files to work, the code I'm using is below...

$count=count($_FILES['photographs']['tmp_name']);

$file_posts=array();

for($i=0;$i<$count;$i++) {
    if(!empty($_FILES['photographs']['name'][$i])) {    
 $_FILES['photographs']['tmp_name'][$i] = "@".$_FILES['photographs']['tmp_name'][$i];
    }
}

$post = array_merge($_POST, $_FILES);

$ch = curl_init();
curl_setopt($ch,CURLOPT_URL,"http://url/to/file.php");      
curl_setopt($ch,CURLOPT_USERAGENT, "Mozilla/4.0 (compatible; MSIE 5.01; Windows NT 5.0)");
curl_setopt($ch,CURLOPT_HEADER,TRUE);
curl_setopt($ch,CURLOPT_POST,TRUE);
curl_setopt($ch,CURLOPT_POSTFIELDS,$post);
curl_exec($ch);
curl_close($ch);

I've tried many variants of this but I can't seem to get files to work no matter what, other post data is fine however.