Send files and json data with fetch

Question

I'm trying to send data (files and json) to the server using fetch with react and redux.

My code is as follows :

export function saveUser(user){
  return function(dispatch){

    fetch(`${data.ROOT_URL}/save-user/`, {
        method: 'post',
        // body: How???
    })
        .then(response => {

            if (response.status !== 200) {
                console.log('Looks like there was a problem. Status Code: ' + response.status);
                return;
            }

            console.log('You are successfully registered!: ' + response.status);
        })

        .catch(function (error) {
            console.log(error);
        });
  }
}

And the user data is as follows :

Thus, I have one big objects with json data and files (photos, documents), but I'm not able to send it to the server.

Any suggestions?

EDIT

After adding

headers: {
      'Content-Type': 'application/json'
},
body: JSON.stringify(user)

The rest of the data is ok, but for the files, where I should get uploaded files, I get only an empty object for the file which is uploaded. If the file isn't uploaded the value of file is null as it should be.

"files":[
    {"file_type":"statutenFile","values":{"active":false,"fileName":"Articles of Association","file":null}},
    {"file_type":"blankLetterheadFile","values":{"active":false,"fileName":"Blank letterhead","file":null}},
    {"file_type":"companyPhotoFile","values":{"active":false,"fileName":"Company photo","file":null}},
    {"file_type":"idCardFile","values":{"active":true,"fileName":"check","file":{}}}
 ]

The last one idCardFile is uploaded but I get "file":{}