The image you see above is from the latest leaked build of 8.1 and it clearly shows that ReFS support is present in 9369. Now, Microsoft could pull the support for the file system before launch of the platform but as of now, it is looking like the updated file system will make its way to Windows 8.1.

What is ReFS? Well, it's a file system that made its debut back in 2012 and builds upon NTFS. The platform brings enhanced support for large volumes and improved reliability for on-disk structures. In addition, the platform ties into existing APIs and "ReFS supports existing Windows and NTFS features such as BitLocker encryption, Access Control Lists, USN Journal, change notifications".