Google API PHP更新文件

我正在尝试更新文件的内容。使用PHP函数：

function updateFile($service, $fileId, $newTitle, $newDescription, $newMimeType, $newFileName, $newRevision) {
    try {
        // First retrieve the file from the API.
        $file = $service->files->get($fileId);
        // File's new metadata.
        $file->setTitle($newTitle);
        $file->setDescription($newDescription);
        $file->setMimeType($newMimeType);
        // File's new content.
        $data = file_get_contents($newFileName);
        $additionalParams = array(
            'newRevision' => $newRevision,
            'data' => $data,
            'mimeType' => $newMimeType
        );
        // Send the request to the API.
        $updatedFile = $service->files->update($fileId, $file, $additionalParams);
        return $updatedFile;
    } catch (Exception $e) {
        print "An error occurred: " . $e->getMessage();
    }
}
....
$data = retrieveAllFiles($service);

$fileName = 'test.txt';
$mimeType = mime_content_type('./'.$fileName);

$res = updateFile($service, $data[0]['id'], $data[0]['title'], 'update', $mimeType, $fileName, true);

我正在尝试添加一个文本文件行"测试字符串"。函数更新数据文件（description，lastModifyingUser…），但文件的内容保持不变。谁能知道出了什么问题？