[dir view] show 'leaving page' confirm popup when a file is uploading (#2612)

static/scripts/app/views/fileupload.js

@@ -49,6 +49,8 @@ define([
                 'canceled': gettext("File Upload canceled"),
                 'failed': gettext("File Upload failed")
             };
+            // for the leaving page confirm popup
+            window.fileuploading = fu_status.uploading;
 
             var uploaded_files = [];
             var updated_files = [];