Improve experience with file uploads

Related: https://github.com/thoughtbot/factory_bot/issues/385

On a current project we have a trait that looks like this:

    trait :with_profile_image do
      transient do
        profile_image_file { Rails.root.join("spec", "fixtures", "jpeg.jpg") }
      end

      after :build do |person, evaluator|
        file = evaluator.profile_image_file

        person.profile_image.attach(
          io: file.open,
          filename: file.basename.to_s,
        )
      end
    end

This seems overly complicated. Maybe we could have done something more like:

    trait :with_profile_image do
      profile_image do
        Rack::Test::UploadedFile.new('spec/fixtures/jpeg.jpg', 'image/jpeg')
      end
    end

But that also seems a little awkward to me, and something I am unlikely to remember.

I would like to at least see some documentation for the best way to handle file uploads in factory_bot, and maybe we could build something into factory_bot or factory_bot_rails to make this easier.

Maybe something like:

    trait :with_profile_image do
      profile_image { file_fixture("jpeg.jpg") }
    end

See https://github.com/rails/rails/blob/b9ca94caea2ca6a6cc09abaffaad67b447134079/activesupport/lib/active_support/testing/file_fixtures.rb#L24, https://github.com/rails/rails/blob/61c4be477706b721688e36a3168f86aabc625658/activestorage/test/test_helper.rb#L82-L84, and https://github.com/rack-test/rack-test/blob/a2f762d5abc2ead277dab51794d22083b72809ea/lib/rack/test/uploaded_file.rb for inspiration

Start by reading the issue discussion and the referenced Rails files, especially active_support/testing/file_fixtures.rb and activestorage/test/test_helper.rb, along with rack/test/uploaded_file.rb. Determine whether the accepted scope is documentation or a new factory_bot/factory_bot_rails helper; done means the supported recommendation and usage are clearly documented, or the agreed helper behavior is implemented and documented.