HEIF is the container format defined by MPEG-H Part 12 (ISO/IEC 23008-12) for still images and image sequences. A file is labelled image/heif when it conforms to the generic 'mif1' brand rather than committing to a particular codec, so the picture inside may be coded with HEVC or something else entirely.
Most people meet the format through Apple devices, but those photos are usually labelled image/heic instead, because they specifically use HEVC coding. If you are unsure which label applies, image/heif is the safer, more general one.
File types
.heif.hif
Also appears as
Not known to appear as any other types