as far that I know, the Plesk Extension AntiVirus hasn't been invented to scan files and folders as you desire it. It is integrated as a mail - handler and nothing more.
Officially Plesk supports two antivirus solution - Kaspersky Anti-Virus and Plesk Premium Antivirus. Both of them are designed to scan mail, but not the file system. Any other antivirus solutions you can install and use at your own risk. Also you can check system with rkhunter utility: