title: Querying by "_id" doesn't return file in MongoDB in Python, using PyMongo
body: <p>I'm using Python + MongoDB + PyMongo in Openshift</p>
  
  <pre><code>import os
  import gridfs
  from django.http import HttpResponse
  from pymongo.connection import Connection
  from django.shortcuts import get_object_or_404, render_to_response
  from django.http import HttpResponseRedirect, HttpResponse
  from django.template import Context, RequestContext,loader
  
  connection = Connection('mongodb://sbose78:XXXXXX@staff.mongohq.com:10068/BOSE')
  db=connection['BOSE']
  fs=gridfs.GridFS(db)
  </code></pre>
  
  <p>When I query a file by _id, this is what I get.</p>
  
  <pre><code>&gt;&gt;&gt; fs.exists({"_id":'504a36d93324f20944247af2'})
  False
  </code></pre>
  
  <p>When I query with the corresponding filename:</p>
  
  <pre><code>&gt;&gt;&gt; fs.exists({"filename":'foo.txt'})
  
  True
  </code></pre>
  
  <p>What could be possibly be going wrong?</p>
  
  <p>Thanks.</p>