application_helper.rb

module ApplicationHelper

  def s3_upload_form_tag(bucket, subfolder = "uploads", presigned_post_options = {}, html_options = {}, &block)
      key = [
        subfolder,
        SecureRandom.urlsafe_base64,
        '${filename}' ].reject(&:blank?).join('/')

      s3 = AWS::S3.new
      bucket = s3.buckets[bucket]
      presigned_post = bucket.objects[key].presigned_post(presigned_post_options)

      html_options[:action] = presigned_post.url.to_s
      html_options[:enctype] = 'multipart/form-data'
      html_options[:method] = 'post'

      content_tag("form", html_options) do
        presigned_post.fields.each do |name, value|
          concat(hidden_field_tag(name, value))
        end
        concat(capture(&block)) if block_given?
      end
    end
  end

end

controller.rb

class StudentCsvUploadsController < ApplicationController
  before_filter :authenticate_user!

  layout :determine_layout

  def new
    @school = current_user.effective_affiliated_schools.find(params[:school_id])
  end

  def show
    @school = current_user.effective_affiliated_schools.find(params[:school_id])
    @student_csv_import = current_user.student_csv_imports.find_by_bucket_and_key(params[:bucket], params[:key])
    if @student_csv_import
      redirect_to school_student_csv_imports_path(@school)
    else
      @student_csv_import = current_user.student_csv_imports.create!(
        bucket:    params[:bucket],
        school:    @school,
        key:       params[:key],
        status:    'pending')
      TTM::QC.enqueue_import("StudentCsvImporter.import", @student_csv_import.id)
      redirect_to school_student_csv_imports_path(@school), notice: "File #{@student_csv_import.basename} has been uploaded and is queued for import."
    end
  end

end

new.html.slim

  h3 Step 3
  p Choose your template from where you saved it on your local computer by clicking on the 'Choose File' option below. Click 'Import'.
  br
  = s3_upload_form_tag S3_UPLOAD_FORM_BUCKET, 'student_csv_uploads', success_action_redirect: school_student_csv_upload_url(school) do
    p= file_field_tag :file
    
    hr   
    p= submit_tag "Import", class: "corebutton blue medium "