[Dovecot] Maildir filename extensions S=<size> and W=<vsize>

I'd like to add S=<size> and W=<vsize> to the filenames in my Maildir storage for newly delivered mails. But I cannot find where I can make Dovecot's LDA add those. Anybody?

http://wiki2.dovecot.org/MailboxFormat/Maildir says:

A maildir filename with those fields would look something like: 1035478339.27041_118.foo.org,S=1000,W=1030:2,S which is exactly what I want, but it doesn't tell me how!

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