新人求助: js如何自动判断gif和jpg

var Gifffer = function() {
    var images, d = document, ga = 'getAttribute', sa = 'setAttribute';
    images = d && d.querySelectorAll ? d.querySelectorAll('[data-gifffer]') : [];
    var createContainer = function(w, h, el) {
        var con = d.createElement('DIV'), cls = el[ga]('class'), id = el[ga]('id');
        cls ? con[sa]('class', el[ga]('class')) : null;
        id ? con[sa]('id', el[ga]('id')) : null;
        con[sa]('style', 'position:relative;cursor:pointer;width:' + w + 'px;height:' + h + 'px;');
        // creating play button
        var play = d.createElement('DIV');
        play[sa]('style', 'width:60px;height:60px;border-radius:30px;background:rgba(0, 0, 0, 0.3);position:absolute;left:' + ((w/2)-30) + 'px;top:' + ((h/2)-30) + 'px;');
        var trngl = d.createElement('DIV');
        trngl[sa]('style', 'width:0;height: 0;border-top:14px solid transparent;border-bottom:14px solid transparent;border-left:14px solid rgba(0, 0, 0, 0.5);position:absolute;left:26px;top:16px;')
        play.appendChild(trngl);
        // dom placement
        con.appendChild(play);
        el.parentNode.replaceChild(con, el);
        return {c: con, p: play };
    }
    var process = function(el) {
        var url, con, c, w, h, play, gif, playing = false, cc, isC;
        url = el[ga]('data-gifffer');
        w = el[ga]('data-gifffer-width');
        h = el[ga]('data-gifffer-height');
        el.style.display = 'block';
        c = document.createElement('canvas');
        isC = !!(c.getContext && c.getContext('2d'));
        if(w && h && isC) cc = createContainer(w, h, el);
        el.onload = function() {
            if(isC) {
                w = w || el.width;
                h = h || el.height;
                // creating the container
                if(!cc) cc = createContainer(w, h, el);
                con = cc.c;
                play = cc.p;
                con.addEventListener('click', function() {
                    if(!playing) {
                        playing = true;
                        gif = d.createElement('IMG');
                        gif[sa]('style', 'width:' + w + 'px;height:' + h + 'px;');
                        gif[sa]('data-uri', Math.floor(Math.random()*100000) + 1);
                        setTimeout(function() {
                            gif.src = url;
                        }, 0);                        
                        con.removeChild(play);
                        con.removeChild(c);
                        con.appendChild(gif);
                    } else {
                        playing = false;
                        con.appendChild(play);
                        con.removeChild(gif);
                        con.appendChild(c);
                        gif = null;
                    }
                });
                // canvas
                c.width = w;
                c.height = h;
                c.getContext('2d').drawImage(el, 0, 0, w, h);
                con.appendChild(c);
            }
        }
        el.src = url;
    }
    for(var i=0; i<images.length; i++) process(images[i]);
}