def METHOD_NAME(
    source_types: List[ResponsiveImageSourceTypes],
    thumbnails: List[ResponsiveImageThumbnail],
) -> Dict[str, str]:
    srcsets = {}
    for source_type in source_types:
        srcsets[source_type.mime_type] = ", ".join(
            [
                "{url}{extension} {width}w".format(
                    url=thumbnail.url,
                    extension=source_type.extension,
                    width=thumbnail.size[0],
                )
                for thumbnail in thumbnails
            ]
        )
    return srcsets