JavaScript 事件不触发

在class上绑定的事件，点击图片的时候无法触发

<div id="files" class="files">
    <div>
        <p>
            <canvas width="80" height="80"></canvas>
            <br><span class="preview"></span><span class="name"></span><span class="size"></span><br>
            <button class="btn btn-primary">Upload</button>
        </p>
    </div>
</div>