Klavissimo doesn't read MIDI files (.mid) — they carry the sound, but no notation: no hands, no rests, no note values. MuseScore opens them and saves them as MusicXML, and from there you import like any other piece. For files from a notation program it takes about a minute.
Open the MIDI file in MuseScore
File → Open → pick your .mid file. MuseScore turns the sound straight into notation; no extra tool needed.
Check what came out of it
Three things are worth a look: are there two staves (right hand in treble, left in bass clef)? Are the time signature and any pickup bar right? Do the note values look plausible — or is it swarming with 32nds and scraps of rests where quarters should be?
Export as MusicXML
File → Export → MusicXML, exactly as for a piece you set yourself. The steps with screenshots are in the MuseScore guide.
Import, then fill in title and tempo
After importing into "My pieces", type the title and tempo in the preview: a MIDI file has no title field, and the tempo often doesn't survive the detour. Both fields are editable right there — the notes themselves arrive complete, including the split between hands for files from a notation program.
Good to know:
Worth knowing before you put in the work: MIDI files come in two kinds. Those exported from a notation program (Finale, Sibelius, MuseScore, commercial sheet-music libraries) sit on the beat and turn into readable notation reliably. Recordings played on an instrument, though — from a digital piano, or "performance" MIDI off the web — have no bar structure, just the player's free timing. Even MuseScore only produces a rough draft from those, with chopped-up note values you'd have to fix bar by bar. For such files it's almost always faster to look for the piece as ready-made MusicXML.
Your data:
This route stays local too: MuseScore runs as a program on your own computer, and imported pieces stay in your browser — nothing from your MIDI file is uploaded.