It option you can enable here: Main menu → Preferences → Configuration → Operation → group "Main program" → select checkbox "Select only the file name when renaming (not the extension)".
Also, to select only the file name without the extension when renaming, you can use the same hotkey that is used to invoke the rename operation. If you press the hotkey again to call the file rename function, i.e. repeatedly press the standard combination of renaming Shift+F6 (or any other combination or button that is configured for this, for example, popular for renaming F2), then the file name will be highlighted in turn, together with the extension and without it, i.e. only the file name.