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!');
}

You could use the validation plugin for jQuery: http://docs.jquery.com/Plugins/Validation

It happens to have an accept() rule that does exactly what you need: http://docs.jquery.com/Plugins/Validation/Methods/accept#extension

Note that controlling file extension is not bullet proof since it is in no way related to the mimetype of the file. So you could have a .png that's a word document and a .doc that's a perfectly valid png image. So don't forget to make more controls server-side ;)

This code works fine, but the only issue is if the file format is other than specified options, it shows an alert message but it displays the file name while it should be neglecting it.

$('#ff2').change(
                function () {
                    var fileExtension = ['jpeg', 'jpg', 'pdf'];
                    if ($.inArray($(this).val().split('.').pop().toLowerCase(), fileExtension) == -1) {
                        alert("Only '.jpeg','.jpg','.pdf' formats are allowed.");
                        return false; }
});

    $("input[name='btnsubmit']").attr('disabled', true);
    $('input[name="filphoto"]').change(function () {
    var ext = this.value.match(/\.(.+)$/)[1];
    switch (ext) 
    {
    case 'jpg':
    case 'jpeg':
    case 'png':
    case 'bmp':
        $("input[name='btnsubmit']").attr('disabled', false);
    break;
    default:
        alert('This is not an allowed file type.');
        $("input[name='btnsubmit']").attr('disabled', true);
        this.value = '';