I also came up with the similar scenario to load the binary data directly using filedata instead of setting file attribute. I did this by passing dummy name in a file attribute along with actual binary data set in filedata and everything works fine.
var xhr = new XMLHttpRequest();
xhr.open('GET', '/test.nii', true);
xhr.responseType = 'arraybuffer';
xhr.send();
xhr.onreadystatechange = function (e) {
if (this.readyState === 4) {
var r = new X.renderer2D();
r.container = 'myImg';
r.orientation = 'Z';
r.init();
volume = new X.volume();
volume.file = "abc.nii";
volume.filedata = this.response;
r.add(volume);
r.render();
}
};