Adding custom attribute to input field

Hi,

I’d like to add a placeholder attribute to certain input fields, and since the form fields are generated through php (why is this?) I can’t simply override the form with a template in my theme folder. So I resorted to trying to hack it into the core file frontend-uploader.php (yeah yeah I know, bad practice but it won’t be updated anyway since it’s for a short-term project) and added it to $atts and included it in the shortcode. It’s in the $atts array, has a value, but it just won’t show up in the output, unlike the class and id in the same array. Is there any other way (a hook for functions.php perhaps?) to solve this?

http://ww.wp.xz.cn/plugins/frontend-uploader/