From https://css-tricks.com/snippets/jquery/automatically-discover-document-links-and-apply-classThis will look through every a element on the page. If the href attribute of it has a .doc, .xls, or .pdf in it, it will apply the appropriate class name to it (e.g. class="doc")

ApplyClassToFileLinks.js

// Will iterate through every 'a' tag element with an 'href' attribute on the page. 
// If the href attribute of it has a .doc, .xls, or .pdf in it, 
// it will apply the appropriate class name to it (e.g. class="doc")

$('a[href]').each(function() {
  if((C = $(this).attr('href').match([.](doc|xls|pdf)$))) {
       $(this).addClass(C[1]);
   }
});