Consider publishing extension-less files in the attachment content family

When attaching files, then as per #896 (closed) and #368 (closed) files without an extension are only published for examples and partials, but not for attachments.

In our case, we attach Javadoc to the documentation site to keep reference docs together with Javadoc. However, Javadoc creates package-list and element-list files that are consumed externally and these are not being published.

I wasn't able to find a config flag to include or publish these files and so while we're publishing Javadoc, other projects cannot reference our documentation site to build their Javadocs.

Reproducer

Repository: https://github.com/spring-projects/spring-data-commons

Command: ./mvnw package -Pantora (Note: Develocity can report scary looking stack traces but that's only log-noise)

Outcome:

1. target/antora/modules/ROOT/assets/attachments/api/java (scan source for Collector Extension) contains element-list
2. target/site/api/java does not contain element-list