When you download a file from the MEGA service, you are shown a pretty download progress bar within the browser. Once this progress bar reaches 100%, your browser then begins to download the file. That is, only once the graphical download is complete, your browser's normal download process is started. What exactly is going on here?
It uses the fileSystem API, which basically writes the file to a sandboxed section of your local file system:
AppData\Local\Google\Chrome\User Data\Default\File System\