Handle the 'cancel' case on attach a file.

chrome/browser/resources/feedback.js

Index: chrome/browser/resources/feedback.js
diff --git a/chrome/browser/resources/feedback.js b/chrome/browser/resources/feedback.js
index 70e63524f7b66d60236c340a6ea8d483d87795e8..e4d6e57ddefe129641757e6b5dc90f2d0677312d 100644
--- a/chrome/browser/resources/feedback.js
+++ b/chrome/browser/resources/feedback.js
@@ -95,6 +95,13 @@ function enableScreenshots() {
  */
 function onFileSelected(evtFileSelected) {
   var file = evtFileSelected.target.files[0];
+  if (!file) {
+    // User canceled file selection.
+    $('attach-file-checkbox').checked = false;
+    attachFileBinaryData = null;
+    return;
+  }
+
   if (file.size > MAX_ATTACH_FILE_SIZE) {
     $('attach-error').hidden = false;