WordPress insert_attachment set post_author

I want to post attachments from the front end in WordPress, here is my code which works:

Form process:

global $post;

if ( $_FILES ) {
    $files = $_FILES['upload_attachment'];
    foreach ($files['name'] as $key => $value) {
        if ($files['name'][$key]) {
            $file = array(
                'name' => $files['name'][$key],
                'type' => $files['type'][$key],
                'tmp_name' => $files['tmp_name'][$key],
                'error' => $files['error'][$key],
                'size' => $files['size'][$key]
            );

            $_FILES = array("upload_attachment" => $file);

            foreach ($_FILES as $file => $array) {
                $newupload = insert_attachment($file,0);
            }
        }
    }
}

Function to process uploads:

function insert_attachment($file_handler,$post_id,$setthumb='false') {
    // check to make sure its a successful upload
    if ($_FILES[$file_handler]['error'] !== UPLOAD_ERR_OK) __return_false();

    require_once(ABSPATH . "wp-admin" . '/includes/image.php');
    require_once(ABSPATH . "wp-admin" . '/includes/file.php');
    require_once(ABSPATH . "wp-admin" . '/includes/media.php');

    $attach_id = media_handle_upload( $file_handler, $post_id );

    if ($setthumb) update_post_meta($post_id,'_thumbnail_id',$attach_id);
    return $attach_id;
}

I would like to set the post_author of the attachment, as currently is defaults to the current logged in user.

I have tried adding the following into the function, but it doesn’t seem to work:

$my_post = array(
    'ID'          => $attach_id,
    'post_author' => 2
);
wp_update_post( $my_post );