Sorry, I have checked before I wrote first post. Based on docs - the path should be in /etc/dovecot/dovecot.conf or /etc/dovecot/conf.d/10-mail.conf (I have Ubuntu 16.04 LTS). I have some familiar lines in 10-mail.conf:

#mail_attachment_dir =

#mail_attachment_min_size = 128k
#mail_attachment_fs = sis posix
#mail_attachment_hash = %{sha1}

but all these lines are commented out. So - in theory - dovecot does not store attachments anywhere but when I log into i.e. Roundcube, I have there mails with attachments. In Maildir/cur I see formatted files related with specific emails. These with attachments have specific hash at the bottom of content. As we could suppose they are hashed by sha1. But still no idea where are real files, which I can download after login to mailbox.