In Play MPE v8, uploading assets is entirely handled through the web browser tools created in the Assets.

Release creators who have asset creation privileges can sign in and access these by clicking on the main menu and selecting Assets.

In Assets you can view all of the songs and playlists available to you that you can then apply to releases. 

Using your web browser you can upload songs and multiple tracks, create and edit metadata for each track and playlist, and assign ownership.