It's weird thing. I tested it on a clean KDE's Neon with english-only locale and fresh Vivaldi installation - files failed to attach.
Also I tested it with clean Chromium and it failed too.
So I think it's not a Vivaldi's fault.
I can confirm that this is also an issue in Windows Release 2.9.1705.41 (windows 10) with the developer tools window undocked.
I can't quite tell from the original post whether the dropdown menu is still usable or not, but in my case, I am not able to select anything from the dropdown when it displays separately as shown in the original message.
As a workaround, I am able to select items from the dropdown if I dock the developer tools within the browser. However, if I have to frequently change the settings and actively use the developer tools for other tasks, this becomes cumbersome.
Bug report VB-60265 has been created relating to this.