I also liked the name additional-files, it felt user-focused and that seems important for pyproject.toml.
Most of the naming conversation has been .dist-info-focused so I wanted to note that there’s utility beyond the files being in .dist-info: this information is also useful to tools that are inspecting source distributions for files of a certain category. The reason files are put into .dist-info/{directory} is so that tools beyond pyproject.toml can add more files as needed without having to change the core metadata of a package.
Throwing a few names out there that I think are also fine?
metadata-filesadditional-metadata
My only thought on extra-* is that the term is already used for something else in Python packaging, so I’d like to avoid adding more uses. My aversion to dist-* or distribution-* is that users don’t think about that word when they think about a Python package, IMO?