Creating a file from a blob

I'm in need of some javascript guru. I have this code:

handleImage(new File([blob], blob.name, {type: blob.type})).done(/* something */)

and

handleImage = function (image) {
        // create some fake form data
        var formData = new FormData();
        formData.append("attachment", image);
        formData.append("auto", true);
        formData.append("_csrf", "xxxxxxxxx");

        // post to the server.
        return $.ajax({
            url: "/some/url",
            data: formData,
            cache: false,
            contentType: false,
            processData: false,
            type: 'POST',
            error: function () {
                console.log("error");
            }
        });

This works fine with Chrome and Firefox, but when using Safari (10.1.1), the server (java / spring mvc) receive in the MultipartHttpServletRequest an empty file for "attachment". So it seems to me that new File([blob], blob.name, {type: blob.type}) is somehow failing.

Any idea of what's wrong here?