Default fallback content type
23.14. Overwriting the default fallback content type for multipart messages By default if no Content-Type header is present in a part, "text/plain; charset=us-ascii" is used as fallback. This is the value defined by the MIME RFC. However for example some web clients (like most, if not all, web browsers) do not send Content-Type headers for all fields in a multipart/form-data request (only for the file parts). This can cause character encoding and unmarshalling errors on the server side.