Thanks man! This is very helpful since shopify does not always provide us with documentation.
I was guided by your post but replaced node.js with ruby on rails and also wrote whole article with flow/logic , implementation and a little bit went into details: https://dev.to/xao0isb/shopify-send-file-to-shop-with-ruby-on-rails-44i7
So anyone who still struggles with understanding on how to upload file to shopify shop can also be guided by the article.