fix: match index files only by using entire basename (#12815)

* fix: match index files only by using entire basename

The previous code would cause file names like `index.xml.ts` to be
treated as index files, when they aren't (the file basename excluding
the extension `.ts` is `index.xml`, and `index` should be the index
file). To match index files only, match the entire first half of the
basename when splitting by the last occurrence of the period/file
extension delimiter.

Pedantically this might not be correct -- i.e. some file extensions
actually span multiple delimiters, like index.tar.gz. But in that case
it's not an index file as well, so I think this is fine.

Fixes #12675

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