Suddenly renaming highlights file name and extension..

I'm used to renaming my files by pressing the return key while the file is selected. IIRC, it used to select just the file name without the extension, and that's what I want it to do. However, for some reason, it now selects everything after pressing the return key, so I have to either de-select the extension or re-type it myself. I didn't change any setting, any idea why is this happening and how can I change it back?

Thanks a lot.

Thanks, but no, they are the extensions.
I'm talking about files such as: .txt, .pdf, .mp3 etc.. For some reason everything including the extension is being selected. It didn't happen before and I have no idea what could trigger such a change.