title: In Scrapy, download files nested below the to of a yielded item dict
body: <p>To download files in Scrapy, one adds the key 'fileurls' to the yielded item dict with a value of the urls to download. But my files are nested somewhere below the top level of the yielded dict. An item looks like this:</p>
  
  <pre><code>{
    "title": "foo",
    "files": {
      "drawings": [
        {
          "caption": "bar",
          "fileurl": "http://foo.com/foo/foo.pdf"
        },
        {
          "caption": "second floor",
          "fileurl": "http://foo.com/foo/bar.pdf"
        }
      ],
      "photos": [
        {
          "caption": "bar",
          "fileurl": "http://foo.com/foo/baz.pdf"
        }
      ]
    }
  }
  </code></pre>
  
  <p>Ideally, I'd like each file downloaded and have scrapy add its "file" element next to the "fileurl". But this does not seem to work automatically.</p>
  
  <p>How can I achieve this? The current version of Scrapy is 1.6.0.</p>