All tutorials talk about uploading with slingshot from client side, is it possible to upload from serverside.
Slingshot is a package specially made to upload files from the client side directly to your cloud storage (S3 or Google Cloud) they made it specifically to fullfill this need because there are enough packages handling server side uploads, they explain that better here.
Maybe it's possible to make it work in the server side by making some mean twists on the code but I wouldn't recommend you to do that because there are already packages that are specially made to handle this.