Native HEIF support in macOS 10.13 High Sierra? I can't find any documentation regarding this. Will HEIF be a natively supported filetype in 10.13?
Per Apple's 10.13 Beta Release Notes:

HEVC and HEIF Images

macOS 10.13 adds HEVC and HEIF decode capability. In order to display HEIF image files or HEVC videos captured on iOS 11, your Mac needs to be updated to macOS 10.13.

The specific hardware and software requirements for HEVC and HEIF are detailed in the WWDC 2017 presentations covering these new technologies. Depending on the capabilities of your Mac and the needs of your content, AVFoundation may use a software-based HEVC encoder which typically has longer encoding times. Further fine-tuning of the encoding quality will occur in subsequent seeds.

Third-party apps that do not rely on the macOS-provided media frameworks may not properly handle these new formats.