Add files.md > File-Type Filters

- resolves notepad-plus-plus#699 item 16

content/docs/files.md

@@ -61,6 +61,12 @@ The **File** menu contains many of the normal file-operation actions found in mo
     - If you have [**Settings > Preferences > Backup**]() `☑ Remember current session for next launch` checkmarked, Notepad++ will save the current list of files to the session.  The only files that will be remembered for the next launch of Notepad++ will be those files that are currently open in Notepad++.
     - If you do not have [**Settings > Preferences > Backup**]() `☐ Enable session snapshot and periodic backup` checkmarked, Notepad++ will prompt you whether to save any files that have been changed, following the same rules as **Close All**, described above.  If you do have that setting checkmarked, unsaved documents will also be included in the session file even if they don't have an underlying file.
 
+### File-Type Filters
+
+For the **Save As** and similar dialogs, there is a **Save as Type** pulldown field.  For the **Open** dialog (and similar), there is a pulldown file-type field to the right of the **File name** entry.  Those pulldowns serve to filter the list of files in the dialog based on the file type.  The filter's list of file types will include all of the languages in the [**Language** menu](../programing-languages/) and the extensions for those types, as defined in the [Style Configurator](../preferences/#style-configurator)). Starting in v8.7, the filter will also include the various [User Defined Language](../user-defined-language-system/) file types and extensions as well.
+
+For the **Save As** dialog, the `☐ Append extension checkbox` described in the [Preference Toggles in Other Dialogs](preferences/#preference-toggles-in-other-dialogs) will allow the dialog to automatically add the _first_ extension in the list for the active filter.  If you want a different extension for that type, or if that option is not checkmarked, you can always type the filename with extension to explicitly define the extension you want for the file you are currently saving.  (So normally, doing **Save As** filtered for C++ will automatically add the `.cpp` extension; but if you want it to be `.cxx` or `.h`, you can just type that as part of the **File name** entry rather than letting Notepad++ auto-append the extension.)
+
 ## Printing
 
 The **File > Print** action will pull up a Windows-standard print dialog, from which you can choose your printer and send your text to the selected printer.  Normally, it will print the whole document, but you can use the print dialog to choose only certain pages; if you have an active selection in the editor, only the selected text will be printed.