title: Getting wagtail to use correct mime types for Documents
body: <p>We have a wagtail (2.0.1) install with documents enabled, but uploading a pdf leads to links to that pdf that don't use the correct mime-type for previewing in the browser. Instead they just instantly go to file download.</p>
  
  <p>Looking at the header, the reply contains:</p>
  
  <pre><code>Content-Type: application/octet-stream
  </code></pre>
  
  <p>which for documents is almost always the wrong mime-type, especially documents that have built-in browser "previews". Is there a way to configure which mime-types to use for which document extensions (settings.py, a mime-type-for-extension admin interface that requires a plugin, etc.)?</p>