How do I fix the ActiveStorage ArgumentError when uploading file attachments in my Rails API?

I'm building a Rails API that uses ActiveStorage to attach files to digital forms. However, when I try to POST a file attachment, I get an error about an unexpected Parameters hash. My routes are configured as follows:

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resources :forms, defaults: { format: :json } do
  resources :attachments, except: [ :update ]
end

My create endpoint in the attachments controller looks like this:

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def create
  @form = Form.find(params[:form_id])

  if params[:attachment].present?
    if @form.update(attachments: params[:form][:attachment])
      render json: { message: "Attachment uploaded successfully", file_url: rails_blob_path(@form.attachments.last, only_path: true) }, status: :created
    else
      render json: form.errors, status: :unprocessable_entity
    end
  else
    render json: { error: "No attachment uploaded" }, status: :unprocessable_entity
  end
end

I have a factory definition for creating forms with attachments:

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FactoryBot.define do
  factory :form do
    user_email { "hello.world@example.com" }
    status { "started" }

    trait :with_attachment do
      after (:create) do |form|
        form.attachments.attach(
          io: Rails.root.join("spec", "files", "upload_test_file.pdf").open,
          filename: "upload_test_file.pdf",
          content_type: "application/pdf"
        )
      end
    end
  end
end

And my spec test is written as follows:

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let(:valid_file) { fixture_file_upload(Rails.root.join("spec", "files", "upload_test_file.pdf"), "application/pdf") }
let(:myform) { FactoryBot.create(:form) }
...
describe "POST /create" do
  it "creates a new Form Attachment" do
    post "/forms/#{myform.id}/attachments", params: { form: { attachment: valid_file } }, headers: valid_headers, as: :json

    myform.reload
    expect(myform.attachments).to be_attached
    expect(response).to have_http_status(:created)
  end
end

The error I encounter is as follows:

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1) /forms/:form_id/attachments POST /create creates a new Form Attachment
   Failure/Error: if @form.update(attachments: params[:form][:attachment])
   
   ArgumentError:
     Could not find or build blob: expected attachable, got #<ActionController::Parameters {"content_type" => "application/pdf", "original_filename" => "upload_test_file.pdf", "tempfile" => "#<File:0x00007f38379ef7c0>"} permitted: false>

I suspect the issue is related to sending the file upload data as JSON (using as: :json in the test), which converts the file into a Parameters hash instead of an instance of ActionDispatch::Http::UploadedFile that ActiveStorage expects. I've reviewed some controller examples and confirmed that my model and factory setups are working as intended. How can I adjust my request or endpoint so that ActiveStorage properly receives the file upload?