Partial templates—like single page templates and list page templates—have a specific lookup order. However, partials are simpler in that Hugo will only check in two places:
Hugo will now also detect the media type and output format of partials, if possible, and use that information to decide if the partial should be parsed as a plain text template or not.
Hugo will look for the name given, so you can name it whatever you want. But if you want it treated as plain text, you should use the file suffix and, if needed, the name of the Output Format. The pattern is as follows:
[partial name].[OutputFormat].[suffix]
Although I am confused by these paragraphs, they are not related to lookup order.
The first reference above is correct and verified through testing. If you want to call partials/ads.amp.html, then you have to:
{{ partial "ads.amp.html" . }}
Try it:
git clone --single-branch -b hugo-forum-topic-35950 https://github.com/jmooring/hugo-testing hugo-forum-topic-35950
cd hugo-forum-topic-35950
hugo server