diff --git a/IPython/nbconvert/writers/files.py b/IPython/nbconvert/writers/files.py
index 70b5b40..a732bd0 100644
--- a/IPython/nbconvert/writers/files.py
+++ b/IPython/nbconvert/writers/files.py
@@ -66,7 +66,7 @@ class FilesWriter(WriterBase):
                 raise AttributeError('notebook_name')
 
             # Pull the extension and subdir from the resources dict.
-            output_extension = resources['output_extension']
+            output_extension = resources.get('output_extension', None)
 
             # Write all of the extracted resources to the destination directory.
             # NOTE: WE WRITE EVERYTHING AS-IF IT'S BINARY.  THE EXTRACT FIG
@@ -101,7 +101,10 @@ class FilesWriter(WriterBase):
                             link_or_copy(matching_filename, dest)
 
             # Determine where to write conversion results.
-            dest = notebook_name + '.' + output_extension
+            if output_extension is not None:
+                dest = notebook_name + '.' + output_extension
+            else:
+                dest = notebook_name
             if self.build_directory:
                 dest = os.path.join(self.build_directory, dest)