Bug 1465458 - Fix launching downloads without a file extension on Windows. r=mak

MozReview-Commit-ID: 6VYjHM46gZx

--HG--
extra : rebase_source : 56e45713138b32adf7f36d9ae65c264e1976be47

toolkit/components/downloads/DownloadIntegration.jsm

@@ -595,7 +595,7 @@ var DownloadIntegration = {
     }
 
     let isWindowsExe = AppConstants.platform == "win" &&
-      fileExtension.toLowerCase() == "exe";
+      fileExtension && fileExtension.toLowerCase() == "exe";
 
     // Ask for confirmation if the file is executable, except for .exe on
     // Windows where the operating system will show the prompt based on the