当表单有file_field输入时，Capybara找不到字段

Question

我的规格有一个非常奇怪的问题。我的表格看起来像这样：

= simple_form_for [:admin, @provider] do |f|
  = f.input :provider_type_id
  = f.input :provider_username
  = f.input :first_name
  = f.input :last_name
  = f.input :athena_health_provider_id
  = f.input :display_name
  = f.input :specialty
  = f.input :entity_type
  = f.input :home_department
  = f.input :billable
  = f.input :npi
  = f.input :ansi_specialty_code
  = f.input :ansi_name_code
  = f.input :languages
  = f.input :description
  = f.input :photo, as: :file
  = f.submit class: 'btn btn-primary'

功能规格看起来像这样：

require 'rails_helper'

feature 'Provider edit' do
  let(:admin)   { create(:user, :admin) }
  let(:provider) { create(:provider) }

  before do
    login_as admin
    visit edit_admin_provider_path(provider)
  end

  context 'when form is filled with invalid data' do
    before do
      fill_in 'First name', with: nil
      click_button 'Update provider'
    end

    scenario 'redirects to :edit' do
      expect(current_path).to eq admin_provider_path(provider)
    end

    scenario 'contains errors' do
      expect(page).to have_text "can't be blank"
    end
  end
end

此规范失败。 Capybara没有使用nil值更新First name输入。但是当我改变输入时：

= f.input :photo

一切都按预期工作。那是为什么？