Part of supporting chrome.downloads in WebExtensions https://developer.chrome.com/extensions/downloads#event-onDeterminingFilename
There's only 95 extensions in Chrome that use this API, so its very sparse, but I do think it will help with some of the concerns around existing Firefox extensions. It would allow extension developers to save files into a directory based on rules determined by the extension. Some of the extensions on bug 1246236 would benefit from this.
Andy thank you for answer! In fact, I do not know any other way to capture the downloading FTP file. Only onDeterminingFilename and it works well in chrome. Our extension captures downloads and helps to download files. Now there is a problem with FTP downloading. If you could add this method to your API in the future it would be very usefull.