Stores a file with the specified base64-encoded contents inside the media folder. Alternatively you can specify a
absolute file path, or a url from where the file shell be downloaded. If more than one of `data`, `path` and `url` are provided, the `data` field will be used first, then `path`, and finally `url`. To prevent Anki from removing files not used by any cards (e.g. for configuration files), prefix the filename with an underscore. These files are still synchronized to AnkiWeb.