function modificaLink() {
  var links = document.getElementsByTagName('a');
  for (i = 0; i < links.length; i++) {
    var link = links[i];
        url = links[i].getAttribute('href');      
      if (url.substring(url.length-3, url.length)=='zip') { 
      icona = document.createElement('img');
      icona.setAttribute('src', '/images/zip.gif');
      icona.setAttribute('alt', 'File zip');
      link.appendChild(icona); 	    
    	}
	else if (url.substring(url.length-3, url.length)=='pdf') {		
	icona = document.createElement('img');
      icona.setAttribute('src', '/images/pdf.gif');
      icona.setAttribute('alt', 'Documento PDF');
      link.appendChild(icona);
	}
	else if (url.substring(url.length-3, url.length)=='mp3') {		
	icona = document.createElement('img');
      icona.setAttribute('src', '/images/mp3.gif');
      icona.setAttribute('alt', 'File MP3');
      link.appendChild(icona);
	}	
  }
}
window.onload = modificaLink;