How do I comment on a Github pull request for a binary file that isn’t shown

Update April 2023:

Commenting on files in a pull request is now generally available (Apr. 2023)

Commenting on files (including deleted, binary, and renamed files) in a pull request is now generally available on the web and GitHub Mobile! A special thank you to everyone that provided feedback during the public beta.

API support is also now available.
See create a review comment (REST API) or addPullRequestReviewThread (GraphQL mutation) for more details on commenting on files.
A new “subject type” field is also now returned by other APIs indicating whether a comment is on a line or file.

Learn more about commenting on a pull request.

Update March 2023:

Comment on files in a pull request (public beta) (Mar. 2023)

Commenting directly on a file in a pull request (not just a specific line) is now available in public beta!

With this capability you can now comment on deleted, binary (including images), and renamed files in a pull request.
You can also comment generally about a changed code file without having to attach the comment to a specific line.

To comment on any file in a pull request, click the Comment on this file button in the header of the file (next to the Viewed checkbox):

Comments on files appear in the Files Changed and Conversation tabs and can be replied to and resolved like regular review comments.

This feature is currently in public beta, with GitHub Mobile and API support coming soon.

Join the discussion and let us know what you think!

2019: First, as illustrated in Reviewable/Reviewable issue 135, this is not possible for GitHub itself (even though it is possible for some third-party tools).

Comments are still evolving (see “Multi-line comments”), but nothing involves binaries.

The main workaround remains to leave a comment on the PR itself, (conversation tab) rather than on a non-supported file.