Params not permitted when using active storage

[benkoshy]

Steps to Reproduce

I have created a minimal rails app - via Github. Here it is: https://github.com/BKSpurgeon/active-storage-test

There is a User model. We make use of the active storage gem to load an 'avatar' of the user.

The problem: in summary, the specs are not passing, when they should.

Expected Behaviour

Both of the following specs should succeed:

## spec/requests/user_spec.rb
require 'rails_helper'

RSpec.describe "Users", type: :request do

  describe "it attaches uploaded files" do
    it 'attaches the uploaded file' do
      file = fixture_file_upload(Rails.root.join('public', 'avatar.jpg'), 'image/jpg')
      expect {
        post api_users_path, params: { user: {username: "Ben", avatar: file } }
      }.to change(ActiveStorage::Attachment, :count).by(1)
    end
  end
end

## spec/acceptance/api/users_controller_spec.rb
require 'acceptance_helper'
require 'action_dispatch/testing/test_process'

resource 'Api' do
  header 'Accept', 'application/json'
  header 'Content-Type', 'application/json'
  header 'Host', 'example.com'

  describe 'POST /api/users' do
    post '/api/users' do
      let(:avatar) { fixture_file_upload(Rails.root.join('public', 'avatar.jpg'), 'image/jpg') }
      let(:username) { 'Test User' }
      let(:params) { {  user: { username: username, avatar: avatar } } }

      context 'with avatar' do
        it { expect { do_request(params) }.to change { ActiveStorage::Attachment.count }.from(0).to(1) }
      endruby 2.6.3p62
    end
  end
end

Actual Behaviour

Only one of the specs pass: the request spec passes, but the controller spec does not pass.

System Configuration

Rails version: 5.2.3
Ruby Version: ruby 2.6.3p62