In Downloads Pro I get the message "(url to file) is a website and not a valid file". Looking at line 42 of /modules/Downloads/functions/mirrors.inc this error occurs when the "Content-Type" header returned by the URL contains the text "xhtml". However, in this case, I fairly sure it doesn't.
Using web-sniffer.net I put in the URL to the file I'm trying to add, and whether I choose GET or POST, or amend any of the other options, the "Content-Type" returned is "application/octet-stream". The status is "HTTP/1.1 200 OK", so I don't know why this is happening.
I could of course comment out the line that produces this error, but I'd rather not.