A file path relative to the Downloads directory to contain the downloaded
file, possibly containing subdirectories. Absolute paths, empty paths,
and paths containing back-references ".." will cause an error.
onDeterminingFilename allows suggesting a filename after the file's
MIME type and a tentative filename have been determined.
Extra HTTP headers to send with the request if the URL uses the HTTPs
protocol. Each header is represented as a dictionary containing the keys
name and either value or
binaryValue, restricted to those allowed by XMLHttpRequest.