private function uploadFile(Bucket $bucket)
{
print "Uploading hello world file to bucket: " . $bucket->getName() . PHP_EOL;
$contents = "Hello world";
$uploadFilename = "hello_world_" . time() . ".txt";
$filepath = __DIR__ . '/upload_file.txt';
file_put_contents($filepath, $contents);
$uploadUrlResponse = $this->m_b2->getUploadUrl($bucket->getId());
$b2File = $this->m_b2->uploadFile($uploadFilename, $filepath, $uploadUrlResponse);
return $b2File;
}