Fix pywwt on notebook >=5.7.6

Version 5.7.6 of the Jupyter notebook module adds a new security feature
related to the HTTP content types
(jupyter/notebook#4467 (comment)). It
breaks pywwt because pywwt has not been returning valid MIME types (AKA
content types) from its internal Jupyter server.

From looking at the main `notebook` implementation, it seems that the correct
approach is to use the `mimetypes` module to get the right value to put in the
Content-Type header, so let's go ahead and do that.

Closes WorldWideTelescope#191.

pywwt/jupyter_server.py

@@ -1,5 +1,6 @@
 import os
 import json
+import mimetypes
 from hashlib import md5
 from tornado import web
 from notebook.utils import url_path_join
@@ -33,6 +34,9 @@ def get(self, filename):
             else:
                 raise web.HTTPError(404)
 
+        # Do our best to set an appropriate Content-Type.
+        self.set_header('Content-Type', mimetypes.guess_type(filename)[0])
+
         with open(path, 'rb') as f:
             content = f.read()