Binary file is bloated/corrupted when downloaded via Node / Express

I'm trying to present a binary file for download via Node and Express.

The file is a Windows binary (.msi) and is around 26MB on disk.

Using Express, I can easily send the file as a download:

res.download('/path/to/file.msi', 'file.msi', function (error) {
    if (error) {
        log.error(error);
    }
});

This seems to work correctly but the resulting downloaded .msi file is almost 100% larger (49MB) and does not execute due to a bad file.

Is there anything else I should be doing to download the file correctly?