HMI is a variant (and incompatible) MIDI format developed by the now-defunct Human Machine Interface Inc. company. It was used by the Sound Operating System, their middleware library for audio playback. They were the main competitor to the Miles Sound System (which used the XMI format) in the game industry in the 90s.
HMI Inc. also developed an earlier midi variant, called HMP, which went through one revision before being succeeded by the HMI format. HMI files can be identified by the HMI-MIDISONG061595 signature at the start of the file.