title: VichUploaderBundle with symfony 4: no extension able to load the configuration for "mappings"
body: <p>I'm trying to use VichUploaderBundle to handle a file upload. I work with symfony 4.
  I have a problem in the first step (Step 1: configure an upload mapping).</p>
  
  <p>in the app/config/config.yml file I have:</p>
  
  <pre><code>vich_uploader:
      db_driver: orm
  
  mappings:
      property_image:
          uri_prefix: /images/properties
          upload_destination: '%kernel.project_dir%/public/images/properties'
  </code></pre>
  
  <p>But it creates an error:</p>
  
  <blockquote>
    <p>(2/2) FileLoaderLoadException
    There is no extension able to load the configuration for "mappings" (in C:\wamp64\www\MonAgence\config/packages/vich_uploader.yaml). 
    Looked for namespace "mappings", found "framework", "sensio_framework_extra", "doctrine_cache", "doctrine", "doctrine_migrations", "security", "swiftmailer", "web_profiler", "twig", "monolog", "debug", "maker", "web_server", "doctrine_fixtures", "knp_paginator", "vich_uploader" 
    in C:\wamp64\www\MonAgence\config/packages/vich_uploader.yaml (which is loaded in resource "C:\wamp64\www\MonAgence\config/packages/vich_uploader.yaml").</p>
  </blockquote>
  
  <p>I searched the internet but I don't see how to fix it.
  Do you have an idea?</p>