A slightly different procedure.
In Download Manager, select the download, then click the trashcan withOUT the red circle.
That stops the download, but it doesn't delete your 80%.
Double-click the new file that came attached with the email. (I hope that's not corrupted too.)
That opens a new Download Manager window.
The new download checks how much is already on your hard drive. This took about ten minutes here to verify what was roughly 54% loaded.
Then the new download resumes at a faster rate.
I've seen as many as ten servers at work (and maybe "it went to eleven" at some point). Currently at 9 servers, but the rate is holding around 670 kB/s.