There is a clear instruction on image upload process:
Upload An Image
Login to the WebUI
Select Tasks -> Start Computer Task
Click Upload on the Computer you created earlier
Boot your Computer via the Client Boot Method you selected
The image upload process will start automatically
If one follows the instruction - client uloads filesystems on every partition it finds. But there is a filesystem (ext4 in my case) without any partitions, right on block device (it makes filesystem size increase very easy and allow to avoid RAID misalignment issues), which it does not ulpoad. How can I make client upload such filesystems too?
The answer was "You can't".
So let it be a feature request.
The feature needed: Client can upload a filesystem situated right on a block device, without any partition label.