Issue/Question: RequestPart and HttpExchange not working together in Tests

First I want to say thank you for all the work that you are doing!

I am not pretty sure if this is an issue or something that I am doing wrong on my side.

I am using @HttpExchange + @PostEchange in addition to @RequestPart for defining HTTP endpoints. When running the application and testing calling the endpoint manually everything works as expected but when I am trying to create a test for the application the multipart functionality stops working.

    @Autowired
    private MultipartfileExampleApplication.OperationsClient operationsClient;

/*
* =====>> This is not working
*/
    @Test 
    void file() {
        var multipartFile = new MockMultipartFile("file", "hello.txt", "text/plain", "Hello World".getBytes());
        var message = operationsClient.multipart(multipartFile);
        Assertions.assertEquals("Hey are here with a multipart request", message);
    }

Reproducible repo

Run the tests in the following repo

https://github.com/Dam14n/multipartfile-example

Research

Again I am considering that what I did was expected. So, there is a real issue that can be solved but If I did something wrong the next suggestion does not make any sense.

In MockMvcClientHttpRequestFactory the request is always created in this way:

			MockHttpServletResponse servletResponse = this.mockMvc
					.perform(request(httpMethod, uri).content(requestBody).headers(requestHeaders))
					.andReturn()
					.getResponse();

I also debugged the internal logic and my findings were that this never take into consideration the files/parts configuration of the request.

So, it would be good to have something like (pseudo-code)

var request = request(httpMethod, uri);
if (request is multipart){
 request = multipart(uri).file(files coming from somewhere??)
}
var finalRequest = request.content(requestBody).headers(requestHeaders)
			MockHttpServletResponse servletResponse = this.mockMvc
					.perform(finalRequest)
					.andReturn()
					.getResponse();

FYI: This was just a suggestion based on my findings which can be 100% a wrong suggestion due to I dont have 100% knowledge of every class/functionality inside Spring boot.

Again thank you Spring team!

Start with MockMvcClientHttpRequestFactory and run the tests in the linked multipartfile-example repository. Trace how the @HttpExchange client request is built and how MockMultipartFile parts are represented, then verify the request reaches the endpoint with its multipart data when tested through MockMvc.