title: Using boto, set content_type on files which are already present on s3
body: <p>I'm using django storages with the s3boto backend.  As per this issue, <a href="http://code.larlet.fr/django-storages/issue/5/s3botostorage-set-content-type-header-acl-fixed-use-http-and-disable-query-auth-by" rel="noreferrer">http://code.larlet.fr/django-storages/issue/5/s3botostorage-set-content-type-header-acl-fixed-use-http-and-disable-query-auth-by</a> I have a bunch of files (all of them) that have content type 'application/octet-stream'.  Given that I have an instance of <code>&lt;class 'boto.s3.key.Key'&gt;</code>, how can I set the content_type?</p>
  
  <pre><code>In [29]: a.file.file.key.content_type
  Out[29]: 'application/octet-stream'
  
  In [30]: mimetypes.guess_type(a.file.file.key.name)[0]
  Out[30]: 'image/jpeg'
  
  In [31]: type(a.file.file.key)
  Out[31]: &lt;class 'boto.s3.key.Key'&gt;
  </code></pre>