If you try using xhr.responseType = "arraybuffer" (jqXHR.xhrFields = { responseType: "arraybuffer" }), the code issues an exception InvalidStateError (in Firefox, when accessing responseText, instead of response):

// Support: IE9
// Accessing binary-data responseText throws an exception
// (#11426)
typeof xhr.responseText === "string" ? {
    text: xhr.responseText
} : undefined

I changed jquery-1.6.1; now I'm going to work with a current version to make a PR or does anyone have something laying around...?