FilenameConflictAction "prompt" not supported?

I’m wondering why the FilenameConflictAction "prompt" (for is not supported for WebExtensions? It was working in the old extensions.

From my point of view, this was the only suitable option for downloading, as you get notified if the file already exist. I don’t want to have the file twice (why should anyone want to have the same file on disk twice? - and even if it is a different file version, the old one is most likely obsolete now). However, simply overwriting the file may also be wrong as one may accidentally overwrite a file which he wanted to keep.

It may be a problem to show prompts from an API call, since it could happen in different contexts, like a background script or a popup. If you want to be sure of the reason, you may want to file a bug report for it.

Chrome and Opera already support it. There should be no reason not to support it on Firefox. It is especially useful for download managers like DownThemAll!.