Uploading a file without using any gems

I want to upload a CSV file from a page, and I also want to resist the urge to install a gem, which seems to be the default Rails developer behavior. No need to build a model; I just need a reference for the CSV for further processing somewhere. Here’s what I did:

First, I built a simple page with a form and a submit button. Ruby on Rails provides some helper methods to generate forms, namely form_with, form_for, and form_tag. I don’t need a model, so I used `form_tag` first. Unfortunately, I ran into this problem where the CSV file handle is not passed to the request. Instead, I used form_with and I was able to parse the CSV.

See also
Source code
FormHelper API documentation
FormHelper Rails Guide
List of file uploading gems