title: Django+Mongodb file upload returns default primary key error
body: <p>I am trying to write a simple file upload application with django + mongodb. I have set my SITE_ID properly (I am writing to the db).</p>
  
  <p>Using the admin page, I can upload a file (and the file appears in the dir structure), but when I click on the filename:</p>
  
  <p><img src="https://i.stack.imgur.com/I80dt.png" alt="filepath"></p>
  
  <p>It gives me this error:</p>
  
  <pre><code>DatabaseError at /admin/myapp/fileUpload/534855062da57ae6ecc4399f/myapp/files/1_2.jpg/
  AutoField (default primary key) values must be strings representing an ObjectId on MongoDB (got u'534855062d...' instead).
  </code></pre>
  
  <p>This is the code I'm using:</p>
  
  <p><strong>models.py</strong></p>
  
  <pre><code>class fileUpload(models.Model):
      some_file_location = models.FileField(upload_to='myapp/files/')
  </code></pre>
  
  <p><strong>admin.py</strong></p>
  
  <pre><code>admin.site.register(fileUpload)
  </code></pre>
  
  <p><strong>DB Entry:</strong></p>
  
  <blockquote>
    <p>db.myapp_fileupload.find()
    { "_id" : ObjectId("53484e4e2da57ad214df5072"), "some_file_location" : "myapp/files/1.jpg" }</p>
  </blockquote>