https://github.com/spring-projects/spring-framework/issues/18551 Solved this issue. If param is an instance of KeyDataPair so add it as part, not as parameter.
Comment From: sbrannen
Thanks for the PR!
We'll review it with the intent to include it in 5.3.
In the interim, @rstoyanchev, in https://github.com/spring-projects/spring-framework/issues/18551#issuecomment-453436578 you mentioned that the implementation would need to switch from a MockHttpServletRequest to a MockMultipartHttpServletRequest; whereas, this PR works directly with the existing Part support in the MockHttpServletRequest. Do you still feel that the HtmlUnit support would need the MultipartHttpServletRequest API?
Comment From: rstoyanchev
Creating MockMultipartHttpServletRequest would be closer to simulating an application with a MultipartPartResolver configured. However, for Servlet 3 multipart requests that's not strictly required, and for example MultipartResolutionDelegate will wrap the request with StandardMultipartHttpServletRequest if necessary.
Comment From: kobaeugenea
So can we leave this way of implementation or should I re-implement to use MockMultipartHttpServletRequest?