title: Submit form using Codeigniter URI Routing not work
body: <p>I have to update a site, that i did not create, with a forum. The site is create with codeigniter's last version. 
  I'm a newbie in routing uri.</p>
  
  <p>When I submit my form, I have an error "404 The page you requested was not found".
  The method add_post() that I want to execute is in the Class Admin_adherent_forum_C. This class is in the controllers/admin_adherent folder.
  There's something wrong with the route but I don't know what? </p>
  
  <p>HTML :</p>
  
  <pre><code>&lt;form action="&lt;?=base_url('espace-adherent/forum/creer_sujet.html')?&gt;" method="post" enctype="multipart/form-data"&gt;
        &lt;div class="CreateSubjectHeader"&gt;
          &lt;p&gt;Nouveau sujet&lt;/p&gt;&lt;a href=".CreateSubject" id="openButtonSubject"&gt;+&lt;/a&gt;&lt;a id="closeButtonSubject"&gt;-&lt;/a&gt;
        &lt;/div&gt;
        &lt;div id="contentSubject"&gt;
          &lt;div class="CreateSubject"&gt;
            &lt;div class="CreateSubjectTitle"&gt;
              &lt;label&gt;Titre :&lt;/label&gt;
              &lt;input type="text" name="sujet" value="" required&gt;
            &lt;/div&gt;
            &lt;div class="CreateSubjectContent"&gt;
              &lt;label&gt;Texte :&lt;/label&gt;
              &lt;textarea name="discussion" required&gt;&lt;/textarea&gt;
            &lt;/div&gt;
            &lt;div class="CreateSubjectCategory"&gt;
              &lt;label&gt;Catégorie :&lt;/label&gt;
              &lt;select id="selectCategory" class="categorie" name="categorie[]" multiple&gt;
                &lt;option value="1"&gt;Entreprise&lt;/option&gt;
                &lt;option value="2"&gt;Collaborateurs&lt;/option&gt;
                &lt;option value="3"&gt;Recyclage&lt;/option&gt;
                &lt;option value="4"&gt;Valorisation&lt;/option&gt;
                &lt;option value="5"&gt;Aménagement&lt;/option&gt;
              &lt;/select&gt;
            &lt;/div&gt;
          &lt;/div&gt;
          &lt;div class="CreateSubjectFooter"&gt;
            &lt;img class="FileIcon" src="/img/file_icon.svg" alt=""&gt;
            &lt;input id="file" class="FileInput" type="file" name="fichier_sujet" value=""&gt;
            &lt;input class="SubmitButton" type="submit" name="publier" value="Publier"&gt;
          &lt;/div&gt;
        &lt;/div&gt;
      &lt;/form&gt;
  </code></pre>
  
  <p>PHP :</p>
  
  <pre><code>defined('BASEPATH') OR exit('No direct script access allowed');
  
    class Admin_adherent_forum_C extends CI_Controller {
     public function add_post()
      {
        my code here...
      }
    }
  </code></pre>
  
  <p>ROUTES :</p>
  
  <pre><code>$route['espace-adherent/forum/creer_sujet']= 'admin_adherent/Admin_adherent_forum_C/add_post';
  </code></pre>