'return the new Content-Type side-effect value'

def apply(self, collection, **kwargs):

    (result, filename) = self._compose(**kwargs)
    mimetype = CONTENT_TYPE(collection)
    if (filename and ((not mimetype) or (CONTENT_TYPE.UNKNOWN == mimetype))):
        (mimetype, _) = mimetypes.guess_type(filename)
        if (mimetype and (CONTENT_TYPE.UNKNOWN != mimetype)):
            CONTENT_TYPE.update(collection, mimetype)
    self.update(collection, *result)
    return mimetype
