Rails 4 parameters; how to whitelist a param to a set of values

I have already read some posts, such as Value whitelist using strong parameters in Rails 4, but it's not quite what I need.

I have a controller which takes an ID and loads a model. It also optionally takes a query string param (style) which can be one of 3 values, small, medium or large. This is passed to a method on the model which uses it to fetch an attached image (using paperclip). I noticed that if I pass an invalid param (eg style=wibble), then I get a 400 error and a notice that the internal file path doesn't exist. Brakeman also notes this as a security issue...

def show
  style = params[:style] || :medium

  thing = Model.find(params[:id])

  path = "#{Rails.root}/public#{thing.image_url(style)}"
  content_type = thing.image.content_type || 'image/png'
  send_file path, type: content_type, disposition: 'inline'
end

We use ActionController Parameters elsewhere to great effect; but I cannot see how that could "whitelist" a parameters options? Everywhere I have seen says to use a model validator, but that's assuming I'm submitting a parameter to update a model, which I am not.

I'm aware I could do something like:

return head :not_found unless %w(small medium large).include? style

Is this the best way?