Generate a hidden_tag when using a file_field

Closes #17947
@egilburg you sure? I looked at the codebase when coding and always find this `stringfy_keys` like https://github.com/rails/rails/blob/master/actionview/lib/action_view/helpers/tags/base.rb#L119 and https://github.com/rails/rails/blob/master/actionview/lib/action_view/helpers/tags/radio_button.rb#L15