For future readers: if you get this problem you can quickly solve it by removing anything from the right of the URL in your address bar up to the .zip and pressing enter.
For example, the following:
https://cgcookie-rails.s3.amazonaws.com/store/download/657/file/cf5545e65ebdcaac2bc05317aaf5964b.zip?response-content-disposition=attachment%3B%20filename%3D"ToyTruck_Start.zip"&X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAITK2SDMAUIGKRQTA%2F20190224%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-Date=20190224T120413Z&X-Amz-Expires=900&X-Amz-SignedHeaders=host&X-Amz-Signature=3fc5ed757f773b253d6efc28d652310923dbdc595a2b5b488d7f54410092ce66
Should be turned into:
https://cgcookie-rails.s3.amazonaws.com/store/download/657/file/cf5545e65ebdcaac2bc05317aaf5964b.zip
I’m not sure why HTML headers are put into url parameters here, but CGCookies webdev team might want to have a look at that.
I can confirm I get this behavior on latest Vivaldi (Chromium/webkit).