Add previewd file extention and type to cache. And clean code.

urls.py

@@ -17,6 +17,7 @@ from seahub.views import root, peers, myhome, \
 from seahub.notifications.views import notification_list
 from seahub.share.views import share_admin
 from seahub.group.views import group_list
+from seahub.utils import set_file_ext_cache
 
 # Uncomment the next two lines to enable the admin:
 #from django.contrib import admin
@@ -108,3 +109,7 @@ if settings.DEBUG:
     urlpatterns += patterns('',
         (r'^%s/(?P<path>.*)$' % (media_url), 'django.views.static.serve', {'document_root': settings.MEDIA_ROOT}),
     )
+
+# Set file extension cache at django startup, this function should be
+# called only once. 
+set_file_ext_cache()