Sending json with files nested in lists with ajax

I am developping a form with some basic inputs and a mini form (in a popup) with a list of foos and each foo have its own attachement file

{

    // form fields
    ...

    foos: [
        {
            // foo form fields
            ...

            attachment: { /* File type */ }
        },
        ...
    ]
}

before i add the attachement property (file upload), everything work good when i submit the whole form with axios to the backend api server (i am using redux-form to manage the form state)

I use JSON.stringify(formValues) to send data with axios as json

But when i add the attachement property i don't know how to send the form because i read that with the file involved in the form i can't no longer send the form as json but I have to use FormData

The problem is I have nested file objects within a list so how can i send the whole form ?

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