title: how to add Active Admin form input :html {multiple => ""}?
body: <p>ActiveAdmin form: </p>
  
  <p>my problem: I can not get a form with multiple choice files(</p>
  
  <p>I tried:  </p>
  
  <pre><code>      ActiveAdmin.register Foto do: 
         form :html =&gt; { :multipart =&gt; true } do |f|
           f.inputs "Upload" do
            f.input :foto, :as =&gt; :file
           end
           f.actions
         end
        end
  </code></pre>
  
  <p>this does not work, then I made a simple html page with two forms:</p>
  
  <pre><code>    &lt;!DOCTYPE doctype html&gt;
      &lt;html&gt;
       &lt;head&gt;
       &lt;/head&gt;
       &lt;body&gt;
        &lt;!--**not** work multiple choice files--&gt;
        &lt;form accept-charset="UTF-8" action="#" enctype="multipart/form-data" method="post"&gt;
          &lt;input id="image" name="image" type="file"/&gt;
        &lt;/form&gt;
  
        &lt;!--**great** work multiple choice files--&gt;
        &lt;form accept-charset="UTF-8" action="#" method="post"&gt;
          &lt;input id="image" name="image" type="file" multiple=""/&gt;
        &lt;/form&gt;
  
       &lt;/body&gt;
      &lt;/html&gt;
  </code></pre>
  
  <p>Question: how to add a property to multiple input field ?</p>
  
  <p>I tried: </p>
  
  <pre><code>     f.input :foto, :as =&gt; :file, :html =&gt; {:multiple =&gt; ""}
       f.input :foto, :as =&gt; :file, :html =&gt; {:multiple =&gt; ""}
       f.input :foto, :as =&gt; :file, :html =&gt; {"multiple" =&gt; "multiple"}
       f.input :foto, :as =&gt; :file, :html =&gt; {:multiple =&gt; :multiple}
  </code></pre>
  
  <p>this does not work</p>
  
  <p>help me, please.</p>