Seems not. Spring boot application will still reject this form submit request with http status code 415.
And with the following error log.
[10-15 12:52:10 WARN ] [o.s.w.s.m.s.DefaultHandlerExceptionResolver] Resolved [org.springframework.web.HttpMediaTypeNotSupportedException: Content type 'text/plain;charset=UTF-8' not supported]
I guest this will only work for these servers which do not check Content-Type header.