For a .mosi file, you can import it from the world pane’s extras menu: press the button with the planet icon twice to get to the world pane, then click the world’s name to open the extras menu - there should be an import button there that opens up an import dialog where you can upload a .mosi file or paste in game data
For an html file you’ll need to open it in a text editor and copy the value of window.GAME_DATA, then paste it into the text field of the import dialog