File Sharing option is available in Plesk interface, but it is marked as "not installed" in Plesk installer
File Sharing functionality is a build-in feature in Plesk, it is neither a component, nor extension that can be removed/installed, so it is not available in Plesk installer.
In Plesk Installer there is "Plesk file server" available under Extensions menu. "Plesk file server" - is not File Sharing, it is an extension called Samba Fileserver Configuration (The File Server module helps share your directories on a Windows network).
Read more about File Sharing functionality here: