You can use gettext filter.
Add following code in your theme’s (child theme’s if you are using) functions.php
add_filter( 'gettext', function( $translated_text, $text, $domain ){
if ( 'front-end-pm' == $domain && 'Add more file' == $text ){
$translated_text = 'add more files';
}
return $translated_text;
}, 20, 3 );