It seems [GitHub serves] gif files under 5MB via raw.githubusercontent. Any files over this size are served as downloadable octet-stream files.
The issue is that the Content-Type for this image is application/octet-stream, which it seems [PyPI's image] proxy can't handle. Instead, it should be image/gif.
80 fixed #77, but now were are hitting https://github.com/pypa/warehouse/issues/5596...