Index: Lib/idlelib/EditorWindow.py
===================================================================
--- Lib/idlelib/EditorWindow.py	(revision 76258)
+++ 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
@@ -600,10 +604,17 @@
         base, ext = os.path.splitext(os.path.basename(filename))
         if os.path.normcase(ext) in (".py", ".pyw"):
             return True
+        if ext and _winreg:
+            try:
+                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
+            except WindowsError:
+                pass
         try:
-            f = open(filename)
-            line = f.readline()
-            f.close()
+            with open(filename) as f:
+                line = f.readline()
         except IOError:
             return False
         return line.startswith('#!') and line.find('python') >= 0