the_sub_field('file_attachment') gets the same thing that get_sub_field('file_attachment'); returns. The only difference is that the_sub_field() echos the value, in this case a stringified array, and get_sub_field() returns the array. If you want to echo the file name then you need to echo the title array element, or one of the other array elements for other things
<?php
// check if the repeater field has rows of data
if( have_rows('attachments') ):
// loop through the rows of data
while ( have_rows('attachments') ) : the_row();
// display a sub field value
$file = get_sub_field('file_attachment');
if( $file ):
?>
<a target="_blank" href="<?php echo $file['url']; ?>"><?php echo $file['title']; ?></a>
<?php endif;
endwhile;
else :
// no rows found
endif;
?>
