Files found in open directories are typically:
In the early days, many system administrators didn't realize their folders were visible to the public. As bandwidth costs rose and security awareness improved, "Directory Browsing" is now disabled by default on most modern web servers. The "Index of" pages are disappearing as the web matures.
When combined, intitle indexof mp4 chennai express top instructs Google: "Find me open, unprotected server directories that list downloadable MP4 video files of the movie Chennai Express." How Open Directories Work