Direct answer: Yes, mid is a type of file. Specifically, a file with the .MID or .MIDI extension is a standard MIDI file that stores musical performance data (note events, timing, velocity, control messages) rather than actual audio data. It is used by music software and hardware to drive synthesizers or sound fonts to generate audio. Context and notes:

• What MIDI files contain: instructions for playback (notes, timing, instrument changes, tempo) rather than sampled audio. This makes them typically much smaller than waveform audio formats like WAV or MP3.
• Variants: Standard MIDI files are commonly Type 0 (single-track data) and Type 1 (multi-track data), with up to 16 channels in a file. The data is organized into events that software interprets to produce sound.
• How they’re used: opened by digital audio workstations (DAWs) and hardware synthesizers, then rendered into audio via a synthesizer or software instrument.