What version are you running?

2.5.3 in production (private instance - access restricted).
Issue also seen on demo.reviewboard.org 3.0 RC2.

What's the URL of the page containing the problem?
No public permalink available.

Can use attachment to see issue on demo.reviewboard.org [Note: may get purged at midnight].
What steps will reproduce the problem?

Create new review request with (attached) cmake.diff.
Go to Diff viewer.

What is the expected output? What do you see instead?
The diff viewer does not highlight the added CMakeLists.txt.

The expected behavior is to see highlighted syntax.
What operating system are you using? What browser?
The private instance is running on Ubuntu 14.04.5 LTS.

The issue is visible on all major browsers: Apple Safari, Google Chrome, Mozilla Firefox, Opera browser. [Haven't tried Microsoft Internet Explorer or Edge.]
Please provide any additional information below.

The file attachment view shows syntax highlighted properly.
The code most relevant to the issue is in f4a7da4ec0f854bf9ae618a3a28d922f704cc90c.