var readChunk = require('read-chunk'); // npm install read-chunk
var fileType = require('file-type');
var buffer = readChunk.sync('unicorn.png', 0, 262);

fileType(buffer);
//=> {ext: 'png', mime: 'image/png'}

var http = require('http');
var fileType = require('file-type');
var url = 'http://assets-cdn.github.com/images/spinners/octocat-spinner-32.gif';

http.get(url, function (res) {
	res.once('data', function (chunk) {
		res.destroy();
		console.log(fileType(chunk));
		//=> {ext: 'gif', mime: 'image/gif'}
	});
});

var xhr = new XMLHttpRequest();
xhr.open('GET', 'unicorn.png');
xhr.responseType = 'arraybuffer';

xhr.onload = function () {
	fileType(new Uint8Array(this.response));
	//=> {ext: 'png', mime: 'image/png'}
};

xhr.send();