So what most probably happened is that FileReader.readAsBinaryString()° defaults to FileReader.readAsText()°° since it's deprecated. At least that is what I saw in Chromium. As soon as I used readAsArrayBuffer the problem went away.

°) https://developer.mozilla.org/en-US/docs/Web/API/FileReader/...

°°) https://developer.mozilla.org/en-US/docs/Web/API/FileReader/...