fix functionality on firefox 50 and 51

frontend/src/upload.js

@@ -80,7 +80,7 @@ $(document).ready(function() {
   if (files.length === 0) {
     toggleHeader();
   } else {
-    for (const index in files) {
+    for (let index in files) {
       const id = files[index].fileId;
       //check if file still exists before adding to list
       checkExistence(id, files[index], true);

frontend/src/utils.js

@@ -1,6 +1,6 @@
 function arrayToHex(iv) {
   let hexStr = '';
-  for (const i in iv) {
+  for (let i in iv) {
     if (iv[i] < 16) {
       hexStr += '0' + iv[i].toString(16);
     } else {