Mmm ahi lo reformé pero me sigue dando el error 400
"message":"File attachment must not be empty","error":"bad_request","status":400,"cause":[]
$url = "https://api.mercadolibre.com/messages/attachments?access_token=".$_SESSION['access_token'];
$cfile = new CURLFile(realpath('c:/prueba.pdf'));
$ch = curl_init();
curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-Type: multipart/form-data'));
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_HEADER, 0);
curl_setopt($ch, CURLOPT_FRESH_CONNECT, 1);
curl_setopt($ch, CURLOPT_FORBID_REUSE, 1);
curl_setopt($ch, CURLOPT_TIMEOUT, 100);
curl_setopt($ch, CURLOPT_POSTFIELDS, array("file" => $cFile));
$art = curl_exec($ch);
print_r($art);