Issue with sending 2 dimensional array of files

I am sending multiple files with formData like this

In my Spring MVC Controller

@PostMapping(value = "/marches")
public Integer saveMarches(
        @RequestPart("formJson") FooBean formJson, 
        @RequestPart("attachOs") MultipartFile[][] attachOs
        ) throws IOException {
    ...
}

My conf :

@Bean(name = "multipartResolver")
public CommonsMultipartResolver multipartResolver() {
    CommonsMultipartResolver multipartResolver = new CommonsMultipartResolver();
    multipartResolver.setMaxUploadSize(30000000);
    return multipartResolver;
}

But i got a 400 Bad Request in the browser

And in the IDE I got :

Resolved [org.springframework.web.multipart.support.MissingServletRequestPartException:
Required request part 'attachOs' is not present]

and if i try @RequestPart("attachOs[][]") MultipartFile[][] attachOs i got always a bad request with Required request part 'attachOs[][]' is not present

The problem is obvious : spring is searching just for attachOs part (@RequestPart("attachOs")) but i am sending attachOs[0][0], attachOs[0][1] ...

When i send just the formJson part without files or if i send just a single file @RequestPart("attachOs") MultipartFile attachOs or one dimension array of files @RequestPart("attachOs") MultipartFile[] attachOs everything works fine.

Javascript code :

const formData = new FormData();

for (const [i, os] of formJson.os.entries()) {
    if(os.attachment) {
        for (const [j, file] of [...os.attachment].entries()) {
            formData.append(`attachOs[${i}][${j}]`, file );
        }
    }
}
...
formData.append('formJson', 
           new Blob([JSON.stringify(formJson)], {type:'application/json'}));
...
axios({
    url: ...,
    method: 'POST',
    data: formData,
})
...

My formJson structure is

{
    // form fields
    ...
    os: [
        {
            // os form fields
            ...
            attachment: [{ /* File type */ }, ...], // multiple files per os
        },
        ...
    ]
}

I know that files cannot be sent along with JSON that's why i am constructing the formData above and after that i will delete the attachment property from JSON structure

So my questions :

1. How to fix the bad request issue ?

2. is there another approach or design pattern to handle this use case ?

from Issue with sending 2 dimensional array of files