What is Google Chrome's "Uncommon Download" warning based on?

Question

I understand that Chrome's "Uncommon Download" warning is broadly based on how common a download is, but what are the specific conditions?

Is "commonness" measured, or is it a heuristic? (eg. "zip files are always considered not common")

If it is measured, is it based on the domain, protocol (eg. http or dataurl), or the full specific URL?

It's clear that file or content type is one of them. From the same website, I've seen that a zip file will trigger the warning, whereas a PNG or JPG won't. Is there any way to make it not trigger for a zip file being created and saved through JSZip?

Pronounced

Do you have an example where the zip download is blocked ? The sample on stuk.github.io/jszip works on my side. – Cholera

Apologies for the rather late response. It's proving a bit difficult to reproduce. It was happening on bulkresizephotos.com but seems to have stopped. When it was happening, it seemed to only happen on Chrome for Windows. I'm wondering if perhaps it was a Chrome bug that crept in and out through auto-updates, as it wasn't present earlier this year, appeared, and now seems to have disappeared again. – Pronounced

@JamesFoster Did you find a fix to this issue? My files are also saying the same. – Kazmirci