Restringir o tipo de arquivos no upload, pelo front-end.

gistfile1.txt

<input type="file"  accept="image/jpg,image/jpeg,image/png,image/gif,image/bmp,image/tiff" />

http://stackoverflow.com/questions/651700/how-to-have-jquery-restrict-file-types-on-upload

Ou 

$('INPUT[type="file"]').change(function () {
    var ext = this.value.match(/\.(.+)$/)[1];
    switch (ext) {
        case 'jpg':
        case 'jpeg':
        case 'png':
        case 'gif':
            $('#uploadButton').attr('disabled', false);
            break;
        default:
            alert('This is not an allowed file type.');
            this.value = '';
    }
});

OU

var ext = $('#my_file_field').val().split('.').pop().toLowerCase();
if($.inArray(ext, ['gif','png','jpg','jpeg']) == -1) {
    alert('invalid extension!');
}

OU

<input type="file" id="userfile" accept="image/*|video/*" required />