var readChunk = require('read-chunk'); // npm install read-chunk
var imageType = require('image-type');
var buffer = readChunk.sync('unicorn.png', 0, 12);

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

var http = require('http');
var imageType = require('image-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(imageType(chunk));
        //=> {ext: 'gif', mime: 'image/gif'}
    });
});

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

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

xhr.send();