So, where I am now: successfully extracted JSon with AttachmentFiles parameters of needed list item. As per all manuals the next step is to put a part of this data to string variable via Get /d/results but I cannot do it - each time my variable comes empty!
I've tried a lot of different variations like /d/results(0)/ServerRelativeUrl and so on but got nothing. I guess the reason is in string variable size limit as per MickB story but don't know how to resolve it.
Appreciate any help!