Spring-test cannot set #RequestPart. It always send it as null - spring-mvc

Question

I cannot test Controller by using mockMvc in spring-test. I want to know right way to test API with #RequestPart.
The method to test is this.
#RequestMapping(value = "/", method = RequestMethod.POST)
public ResponseEntity<Object> replaceFile(
#RequestPart("files") Map<String, Object> files,
#RequestPart("fileKey") String fileKey)
And to test I build a mock request like below.
MockMultipartFile blob = new MockMultipartFile("files", files.getBytes());
MockMultipartFile key = new MockMultipartFile("fileKey", fileKey.getBytes());
mockMvc.perform(fileUpload("/")
.file(blob)
.file(key))
.andDo(print())
.andExpect(status().isOk());
As you can see, I used fileUpload. But in past time, I tried to use post with content or requestAttr because all of them didn't work.
I think current code is the closest to answer among I tried, but can't get closer anymore.
The weird thing is, the real in-use API is almost same with them.
In client side, user sent a new FormData() object to request and server can get data properly.
The server side code is below.
#RequestMapping(value = "/api/{variable}", method = RequestMethod.POST)
public ResponseEntity<Object> apiMethod(
#PathVariable int variable,
#RequestPart("dto1") DTO1 dto1,
#RequestPart("dto2") DTO2 dto2,
#RequestPart("file") Map<String, Object> file)
"file" part consists of "file name" key and its blob value encoded base64.
For example, {"hello.txt": "SGVsbG8gd29ybGQh"}
What I want to know - Right way to test API with #RequestPart
What I tried
method - fileUpload / data - file, content, requestAttr but they send null.
method - post / data - file, content, requestAttr but they throw MultipartException.
Restriction - Cannot use multipart because the system is using a low version of Spring.
Thanks!

Answer 1

Thanks to #borino who commented to my question, I got the clue of problem.
It is certain to use fileUpload for testing API with #RequestPart arguments.
In that case, it caused from content type.
I declared MockMultipartFile without contentType.
MockMultipartFile blob = new MockMultipartFile("files", files.getBytes());
MockMultipartFile key = new MockMultipartFile("fileKey", fileKey.getBytes());
But the arguments of API have a type, Map<String,Object> and String each.
As #borino said to me, I changed constructor of MockMultipartFile to make sure contentType, and it works!
MockMultipartFile blob = new MockMultipartFile("files", "", "application/json", files.getBytes());
MockMultipartFile key = new MockMultipartFile("fileKey", "", "application/json", fileKey.getBytes());
Just add contentType when you have a problem like me. Thanks!