Thank you for your reply. I should have attached the output from the command in the first place, sorry!
pi@raspberrypi:~ $ mopidy local scan
INFO Starting Mopidy 2.1.0
INFO Loading config from builtin defaults
INFO Loading config from /home/pi/.config/mopidy/mopidy.conf
INFO Loading config from command line options
INFO Enabled extensions: iris, mpd, http, file, stream, mobile, m3u, simple-webclient, local-images, softwaremixer, moped, musicbox_webclient, party, api_explorer, local
INFO Disabled extensions: none
INFO Found 4 files in media_dir.
INFO Checking 0 tracks from library.
INFO Removing 0 missing tracks.
INFO Found 4 tracks which need to be updated.
INFO Scanned 4 of 4 files in 0s.
INFO Done scanning.
This is the output after transferring 4 test mp3 files via WinSCP and restarting mopidy.
The folder I used to transfer to was /home/pi/Music, which is the file named as media_dir in [local] section of the config file.