Issues with local data

[Engin61]

I have a "little" problem with local data on Spotify. I have about 20 songs in my playlist which can't be found on Spotify due to being a remix from songs or just because for the fact the the artist is not on Spotify.

I can't play any of these songs on the Web App or other computers, I can only play it on my computer and my HTC because I downloaded it, obviously.

My question now is: How can I make it happen that I can listen to those songs on other computers and the Web App?

Many thanks in advance.