Update UploadHandler.php

In the post() method, if the `$upload` variable is null (`$_FILES[$this->options['param_name']]` does not exist), the old code will still try to handle the upload as if `$_FILES` were a one-dimensional array.
The solution is to split the initial condition:

Original code:
```
if ($upload && is_array($upload['tmp_name'])) {
...
}
} else {
...
}
```

New code:
```
if ($upload) {
    if (is_array($upload['tmp_name'])) {
    ...   
    } else {
    ...
    }
}
```