You can finally say goodbye to that desktop Music Manager app for Google Play Music. Well, as long as you don't mind venturing into the Play Music labs. Google has added a new Chrome app toggle in the labs that enables drag-and-drop music uploads and a cool little pop-out player interface.
Just head to the labs page and enable "Google Play Music for Chrome" and save your changes. Chrome will download the extension, and then you can drag any compatible song files into the Play Music window to upload.