[Bug]: Inconsistent behaviour in save dialog when Windows has file extensions hidden

[wartab]

Preflight Checklist

• I have read the Contributing Guidelines for this project.
• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the issue tracker for a bug report that matches the one I want to file, without success.

Electron Version

29.1.5

What operating system are you using?

Windows

Operating System Version

Windows 10

What arch are you using?

x64

Last Known Working Electron version

never

Expected Behavior

On Windows, when file extensions are hidden, when the user changes the filename, the original extension should be added back. This is the default behaviour when using the browser save dialog.

Actual Behavior

The file extension is removed when the user hasn't typed an extension.

Note that, sometimes, this even happens when the user hasn't changed the file name at all.

Testcase Gist URL

https://gist.github.com/db79d1744481ab377ea8c0904d5e1633

Additional Information

In this Gist, when you click Browser Download, first of all, the save dialog auto-detects the type from system settings:

Changing the name to "i-wanted-another-name":

it'll automatically add back .csv

While with the Electron dialog, we get this (when defaulting to some-file.csv):

Changing the name does this:

{
canceled: false,
filePath: 'C:\\Users\\Vincent\\Downloads\\i-wanted-some-other-name'
}

[MafistoPL]

Isn't it duplicate of 38794?