Bug 1773907, don't clear the extension on the possible filename if we don't...

Bug 1773907, don't clear the extension on the possible filename if we don't have one to replace it with, r=mhowell, a=dmeehan

The issue here is that ShouldModifyExtension returns Replace because the content type isn't known.

Differential Revision: https://phabricator.services.mozilla.com/D151427