nginx: Add PDF files to the content-types list.

Previously, user-uploaded PDF files were not properly rendered by
browsers with the local uploads backend, because we weren't setting
the correct content-type.

puppet/zulip/files/nginx/zulip-include-frontend/uploads.types

@@ -2,6 +2,8 @@
 types {
     text/plain                            txt;
 
+    application/pdf                       pdf;
+
     image/gif                             gif;
     image/jpeg                            jpeg jpg;
     image/png                             png;