Currently not considering the use of mail panel, we can only choose either saving file or sending email. We cannot do both. The original purpose for file saving is to support debugging/logging/testing. Even if we do send out emails, such requirement may still exist. That's why I suggested we turn `useFileTransport` into `enableFileTransport`.

Yes, if we do so we need to adjust the code a bit so that the event handler can know the file name.