title: Rails ActiveStorage Error - MessageVerifier-InvalidSignature
body: <p>I'm working on a project that requires an <code>ActiveStorage</code> <code>has_many_attached :photos</code> situation on a <code>Location</code> model.</p>
  
  <p>I have the code set up below, but when attempting to upload a form, I receive the following error:</p>
  
  <pre><code>ActiveSupport::MessageVerifier::InvalidSignature in 
                                   LocationsController#attach_photo
  </code></pre>
  
  <p>Is this the way to "add" a file to the set of attachments for a particular parent record (i.e: a <code>Location</code> record)?</p>
  
  <h3><code>Location</code> Model</h3>
  
  <pre class="lang-rb prettyprint-override"><code>class Location &lt; ApplicationRecord
    ...
    has_many_attached :photos
    ...
  end
  </code></pre>
  
  <h3>Locations Controller</h3>
  
  <pre class="lang-rb prettyprint-override"><code>class LocationsController &lt; ApplicationController
    ...
    def attach_photo
      @location = Location.find(params[:id])
      @location.photos.attach(params[:photo])
      redirect_to location_path(@location)
    end
    ...
  end
  </code></pre>
  
  <h3>View</h3>
  
  <pre><code>&lt;%= form_tag attach_photo_location_path(@location) do %&gt;
    &lt;%= label_tag :photo %&gt;
    &lt;%= file_field_tag :photo %&gt;
  
    &lt;%= submit_tag "Upload" %&gt;
  &lt;% end %&gt;
  </code></pre>
  
  <h3>View</h3>
  
  <pre class="lang-rb prettyprint-override"><code>resources :locations do
    member do
      post :attach_photo
    end
  end
  </code></pre>