问如何在使用<input type =“file”>时限制文件格式？

var file = document.getElementById('someId');

file.onchange = function(e){

    var ext = this.value.match(/\.([^\.]+)$/)[1];

    switch(ext)

    {

        case 'jpg':

        case 'bmp':

        case 'png':

        case 'tif':

            alert('allowed');

            break;

        default:

            alert('not allowed');

            this.value='';

    }

};

<form>

  <input type="file" name="pic" id="pic" accept="image/gif, image/jpeg" />

</form>