UserDefinedFormController sent email with attachment that is broken

Silverstripe Version: 4.9 ~ 4.11

Question: After upgrading from 4.8 to 4.11, the email sent from the UserDefinedFormController always with the attachment as 0 bytes if the user uploads a file. If it is an image, it can’t be opened. Is there any way to resolve this problem?

I’m wondering if it is because the SwiftMailer has been upgraded to 6.x and has conflicts with the framework or not.

Thanks a lot!