js: fix selectors

public/js/app.js

@@ -17,12 +17,11 @@
 // <http://www.gnu.org/licenses/>.
 
 (function(){
-  var submit = document.getElementById('submit');
+  const submit = document.querySelector('input[type=submit]');
   if (!submit) return;
-
   submit.disabled = true;
 
-  var maxSize = document.getElementsByClassName('max-size');
+  const maxSize = document.querySelector('.max-size');
 
   function checkSize(e) {
     console.log(e.target.files);
@@ -34,7 +33,8 @@
       maxSize[0].classList.add('error');
     }
   }
-  var filepicker = document.getElementById('file');
+
+  const filepicker = document.querySelector('input[type=file]');
   if (filepicker) {
     filepicker.addEventListener('change', checkSize, false);
   }