Prevent wrong content type for CSS files

The mime-util library contains two content type mappings for .css
files: application/x-pointplus and text/css.  Unfortunately, using
the wrong one will result in most browsers discarding the file as
a CSS file.  Ensure we only use the correct type for CSS files.

This happens because MimeUtilFileTypeRegistry attempts to get all
MIME types from mime-util and then sort the result based on the
specificity of each type.  Since both types have no magic string,
only the ExtensionMimeDetector matches.

Change-Id: Idfe88dc823f191d9c9e0b9c9da3b5d2ec471f9db

gerrit-httpd/src/main/java/com/google/gerrit/httpd/plugins/HttpPluginServlet.java

@@ -580,6 +580,9 @@ class HttpPluginServlet extends HttpServlet
       if ("application/octet-stream".equals(contentType)
           && entry.getName().endsWith(".js")) {
         contentType = "application/javascript";
+      } else if ("application/x-pointplus".equals(contentType)
+          && entry.getName().endsWith(".css")) {
+        contentType = "text/css";
       }
     }