How to have jQuery restrict file types on upload?

You can get the value of a file field just the same as any other field. You can’t alter it, however.

So to superficially check if a file has the right extension, you could do something like this:

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