Change var to let for all variables

static/upload.js

-var uploadStatusDiv = document.getElementById("uploadStatusDiv");
-var uploadText = document.getElementById("uploadText");
-var uploadButton = document.getElementById("uploadButton");
-var uploadProcessingMessage = document.getElementById("uploadProcessingMessage");
-var uploadFilename = document.getElementById("uploadFilename");
-var uploadFileBytes = document.getElementById("uploadFileBytes");
-var uploadFileInput = document.getElementById("file");
+let uploadStatusDiv = document.getElementById("uploadStatusDiv");
+let uploadText = document.getElementById("uploadText");
+let uploadButton = document.getElementById("uploadButton");
+let uploadProcessingMessage = document.getElementById("uploadProcessingMessage");
+let uploadFilename = document.getElementById("uploadFilename");
+let uploadFileBytes = document.getElementById("uploadFileBytes");
+let uploadFileInput = document.getElementById("file");
 
 function humanFileSize(bytes, si = true, dp = 2) {
     const thresh = si ? 1000 : 1024;
@@ -29,10 +29,10 @@ function humanFileSize(bytes, si = true, dp = 2) {
 }
 
 function uploadFile() {
-    var formdata = new FormData();
+    let formdata = new FormData();
     formdata.append("file", uploadFileInput.files[0]);
     formdata.append("source", "web");
-    var ajax = new XMLHttpRequest();
+    let ajax = new XMLHttpRequest();
     ajax.upload.addEventListener("progress", progressHandler, false);
     ajax.addEventListener("load", responseHandler, false);
     ajax.addEventListener("error", responseHandler, false);
@@ -44,7 +44,7 @@ function uploadFile() {
 }
 
 function progressHandler(event) {
-    var percent = (event.loaded / event.total) * 100;
+    let percent = (event.loaded / event.total) * 100;
 
     if (percent >= 100) {
         uploadStatusDiv.hidden = true;
@@ -61,4 +61,4 @@ function progressHandler(event) {
 
 function responseHandler(event) {
     document.open('text/html').write(event.target.response).close();
-}
\ No newline at end of file
+}