Add file to galleys using REST API

Hello.
I’m trying to import items to OJS while using REST API and I ran into a problem.
I can’t find a mechanism in API which will let me “convert” publication files to publication galleys…
When retriving a publication with API, galleys “array” is empty, so there is no galley “id” which I could
use as “assocId” value in submission file properties.
Is there any chance that I missed something in documentation or there is no way that I could import
items (with files) to OJS and publish them using only API?

I’m using OJS 3.3.0.10

Thank you for any help or information.