Comment 5 for bug 1642027

See this discussion:

https://discuss.pixls.us/t/linux-applications-and-their-non-standard-xmp-sidecar-naming-convention/2688

In short, I will not be modifying Rapid Photo Downloader to handle the use case you present. You can use ExifTool or exiv2 to move or rename your files that you've already downloaded onto your computer and used Darktable to process.