Open/save dialog assumes the defaultPath parameter without '/' as a default file name.

chrome/browser/resources/file_manager/js/file_manager.js

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * FileManager constructor.
  *
  * FileManager objects encapsulate the functionality of the file selector
  * dialogs, as well as the full screen file manager application (though the
  * latter is not yet implemented).
  *
  * @constructor
  * @param {HTMLElement} dialogDom The DOM node containing the prototypical
  *     dialog UI.
  */
 function FileManager(dialogDom) {
   this.dialogDom_ = dialogDom;
   this.filesystem_ = null;
   this.params_ = location.search ?
                  JSON.parse(decodeURIComponent(location.search.substr(1))) :
                  {};
-  if (this.params_.defaultPath && this.params_.defaultPath.indexOf('/') != 0)
-    this.params_.defaultPath = '/' + this.params_.defaultPath;
-
   this.listType_ = null;
   this.showDelayTimeout_ = null;
 
   this.selection = null;
 
   this.butterTimer_ = null;
   this.currentButter_ = null;
   this.butterLastShowTime_ = 0;
 
   this.filesystemObserverId_ = null;
@@ skipping 1465 matching lines @@
    */
   FileManager.prototype.setupCurrentDirectory_ = function(pageLoading) {
     var path = location.hash ?  // Location hash has the highest priority.
         decodeURI(location.hash.substr(1)) :
         this.params_.defaultPath;
 
     if (!pageLoading && path == this.directoryModel_.getCurrentDirPath())
       return;
 
     if (!path) {
-      this.directoryModel_.setupDefaultPath();
-      return;
+      path = this.directoryModel_.getDefaultDirectory();
+    } else if (path.indexOf('/') == -1) {
+      // Path is a file name.
+      path = this.directoryModel_.getDefaultDirectory() + '/' + path;
     }
 
     // In the FULL_PAGE mode if the hash path points to a file we might have
     // to invoke a task after selecting it.
     // If the file path is in params_ we only want to select the file.
     var invokeHandlers = pageLoading && !this.params_.selectOnly &&
         this.dialogType_ == FileManager.DialogType.FULL_PAGE &&
         !!location.hash;
 
     if (DirectoryModel.getRootType(path) == DirectoryModel.RootType.GDATA) {
@@ skipping 3126 matching lines @@
             this.gdataSpaceInfoBar_.style.width =
                 (100 * usedSpace / result.totalSizeKB) + '%';
           } else {
             this.gdataSpaceInfoBar_.style.display = 'none';
             this.gdataSpaceInfoLabel_.textContent =
                 str('GDATA_FAILED_SPACE_INFO');
           }
         }.bind(this));
   }
 })();