Downloading a file confuses Badger into thinking the tab navigated to the file's URL

[ghostwords]

This can result in site breakage. For example, downloading an attachment in FastMail (fastmail.com) from fastmailusercontent.com results in Privacy Badger considering subsequent fastmail.com resources to be tracking requests.

Initiating a file download seems to produce a main_frame-type request in chrome.webRequest listeners. This is a problem since Privacy Badger considers main_frame requests to designate navigation boundaries (which we know leads to other attribution problems: #1997).

File downloads seem to produce main_frame-type requests in both Chrome and Firefox.

Ideally we would just have direct, synchronous access to the top-level/tab URL that initiated the request for every request (crbug.com/856766) so that we could easily and properly attribute requests.

Failing that, we need a way to distinguish between genuine navigation and file download requests.