You'll need to have the Spotify application installed rather than the Web browser.

Launch the program and make sure you're logged into your account.

Click on Spotify and then preferences.

Ah, pop up window will appear.

Scroll down until you get to the local files section and click this button to allow these files to be shown on the Spotify app on the left hand side.

A local files name will now appear.

Click on it and you'll see lots of audio files from your computer listed.

Now go to your finder on Mac or my computer on Windows and find the file you want to add.

Copy the file and paste it in your downloads folder back in Spotify.

If we scroll down the local files list, you'll see it appear in the bottom.

Click on the three dots icon on the track and select.

Add to playlist and choose one.