No problem to convert to blob object in the first step:
function dataURItoBlob (base64Data) {

var byteString;
if (base64Data.split(",")[0].indexOf("base64") >= 0){
  byteString = atob(base64Data.split(",")[1]);
}else{
  byteString = unescape(base64Data.split(",")[1]);
}
var mimeString = base64Data.split(",")[0].split(":")[1].split(";")[0];
var ia = new Uint8Array(byteString.length);
for (var i = 0; i < byteString.length; iPP) {
    ia[i] = byteString.charCodeAt(i);
}
return new Blob([ia], {type:mimeString});

}
but the second step does not feel right to put the blob object into the formdata:

var fd = new FormData(obj);
var blob = dataURItoBlob (base64,"mimeString");
fd.append("files[]", blob);
ajax405