diff --git a/lib/matplotlib/backend_bases.py b/lib/matplotlib/backend_bases.py
index 4aec8a0fef0f..ba75a447795a 100644
--- a/lib/matplotlib/backend_bases.py
+++ b/lib/matplotlib/backend_bases.py
@@ -2267,7 +2267,7 @@ def get_default_filename(self):
         a default filename.
         """
         default_basename = self.get_window_title() or 'image'
-        default_basename = default_basename.lower().replace(' ', '_')
+        default_basename = default_basename.replace(' ', '_')
         default_filetype = self.get_default_filetype()
         default_filename = default_basename + '.' + default_filetype