Index: Lib/idlelib/EditorWindow.py
===================================================================
--- Lib/idlelib/EditorWindow.py	(revision 74581)
+++ Lib/idlelib/EditorWindow.py	(working copy)
@@ -17,6 +17,10 @@
 from configHandler import idleConf
 import aboutDialog, textView, configDialog
 import macosxSupport
+try:
+    import _winreg
+except ImportError:
+    _winreg = None
 
 # The default tab setting for a Text widget, in average-width characters.
 TK_TABWIDTH_DEFAULT = 8
@@ -600,6 +604,11 @@
         base, ext = os.path.splitext(os.path.basename(filename))
         if os.path.normcase(ext) in (".py", ".pyw"):
             return True
+        if _winreg:
+            with _winreg.OpenKey(_winreg.HKEY_CLASSES_ROOT, ext) as key:
+                ftype = _winreg.QueryValueEx(key, None)[0]
+                if ftype.lower() in ('python.file','python.noconfile'):
+                    return True
         try:
             f = open(filename)
             line = f.readline()