It still work but some file type never return anything, I have the same problem with epub, pretty sure it's some google's shenanigan about books piracy.

https://support.google.com/webmasters/answer/35287?hl=en

That explains why essentially no DejaVu files show up. I was shocked to test out a .djvu search and find almost nothing.

Maybe Google doesn’t index epub at all? I think I never saw one in search results.

Well, I may have become crazy but i have vivid memory using it in the past, and some websites even refer to this specific query ( https://ebookfriendly.com/google-search-tips-books/ )

For some file extensions yes, for others no or not anymore (eg .js)