With this you get the extension, too:
get it:
$path_info = pathinfo( get_attached_file( $attachment_id ) );
show it:
echo $path_info['extension']
full example with a repeater field (to show some downloads):
<?php
// Downloads repeater field
if (get_row_layout() == 'download_list'): ?>
<?php if(get_sub_field('download')) { ?>
<ul class="unstyled downloads">
<?php while(has_sub_field('download')) {
$attachment_id = get_sub_field('singledl');
$url = wp_get_attachment_url( $attachment_id );
$title = get_the_title( $attachment_id );
$filesize = filesize( get_attached_file( $attachment_id ) );
$filesize = size_format($filesize, 2);
$path_info = pathinfo( get_attached_file( $attachment_id ) );
?>
<li class="table">
<a href="<?php echo $url; ?>" class="download table-row break">
<span class="icon-download2 iconF icon-20 icon-fl" aria-hidden="true"></span>
<?php echo '<span class="filetitle table-cell">' . $title . ' (' . $path_info['extension'] . ')</span>' . '<span class="filesize table-cell">' . $filesize . '</span>' ?>
</a>
</li>
<?php } ?>
</ul>
<?php } ?>
<?php ?>
