Fix upload.js existing attachment (#3376)

frappe/public/js/frappe/upload.js

@@ -194,15 +194,17 @@ frappe.upload = {
 		$(document).on('upload_complete', on_upload);
 
 		function upload_next() {
-			i += 1;
-			var file = files[i];
-			args.is_private = file.is_private;
+			if(files) {
+				i += 1;
+				var file = files[i];
+				args.is_private = file.is_private;
+				frappe.show_progress(__('Uploading'), i+1, files.length);
+			}
 			frappe.upload.upload_file(file, args, opts);
-			frappe.show_progress(__('Uploading'), i+1, files.length);
 		}
 
 		function on_upload(e, attachment) {
-			if (i === files.length - 1) {
+			if (!files || i === files.length - 1) {
 				$(document).off('upload_complete', on_upload);
 				frappe.hide_progress();
 				return;