Add a few more comments about dataset path rewriting in the dataset wrapper.

lib/galaxy/tools/wrappers.py

@@ -209,13 +209,21 @@ class DatasetFilenameWrapper( ToolParameterValueWrapper ):
 
     def __getattr__( self, key ):
         if self.false_path is not None and key == 'file_name':
+            # Path to dataset was rewritten for this job.
             return self.false_path
         elif self.false_extra_files_path is not None and key == 'extra_files_path':
+            # Path to extra files was rewritten for this job.
             return self.false_extra_files_path
         elif key == 'extra_files_path':
             try:
+                # Assume it is an output and that this wrapper
+                # will be set with correct "files_path" for this
+                # job.
                 return self.files_path
             except AttributeError:
+                # Otherwise, we have an input - delegate to model and
+                # object store to find the static location of this
+                # directory.
                 try:
                     return self.dataset.extra_files_path
                 except exceptions.ObjectNotFound: