Fix incorrect file reference in `onDownloadDragStart`

Follow-up to Janek's port in 462332eee018e24d88255c708fa8acb67a717673
where this variable was changed creating a mismatch with surrounding
code.

This fixes #943.

application/palemoon/components/downloads/content/downloads.js

@@ -1038,9 +1038,9 @@ const DownloadsView = {
     }
 
     // We must check for existence synchronously because this is a DOM event.
-    let file = new FileUtils.File(DownloadsView.controllerForElement(element)
-                                               .download.target.path);
-    if (!file.exists()) {
+    let localFile = new FileUtils.File(DownloadsView.controllerForElement(element)
+                                                    .download.target.path);
+    if (!localFile.exists()) {
       return;
     }