the URL to download
prompt user for a filename
Construct an object URL string from a given data URL
This is needed because feeding a data URL directly to downloads.download() causes "Error: Access denied for URL"
the URL to make an object URL from
object URL that can be fed to the downloads API
Download a given URL to disk
Normal URLs are downloaded normally. Data URLs are handled more carefully as it's not allowed in WebExt land to just call downloads.download() on them