I should have mentioned before, but forgot, that Windows, by default, hides file name extensions in File Explorer.  (The 'type' column supposedly makes them redundant.)  If they are hidden, they are also hidden in the Save As name box, as described for MacOS.  So I think this is worth a sentence in the doc.

After 4 years, I want to close this with or without an edit (for this and/or #4832).  The current SaveAs entry is

   Save the current window with a Save As dialog.  The file saved becomes the
   new associated file for the window.

(Ned has previously said leave this alone for both issues.)  Proposal:

Save the contents of the current window with a Save As dialog.  (If your file manager is set to hide extensions, the current extension will be omitted in the file name box.)  If the new filename has no '.', '.py' and '.txt' will be added for Python and text files.  (On MacOS Aqua, '.py' is added for any type.)  The file saved becomes the new associated file for the window.

Opinions?