Fails with with illegal character message

Recently this extension stopped working for me. When I click the QC icon I get the following message in the browser console:

Error: filename must not contain illegal characters

from the following line

browser.downloads.download(downloadSettings).then(null, (error) => console.log(error));

I was able to step into the addon with the debugger and change the filename manually. Trying to save a filename of dog.url failed, while saving a filename without an extension, dog, succeeded.

So it appears that the . is causing issues, though I don't know why. I haven't been able to find any known Firefox bugs around this on their bugzilla instance or anything very relevant in the recent release notes.