Should I design my API to receive JSON as a hierarchy child? - forms

Question

I'm designing an API that receives a form post request from the front end. The front-end form contains both regular form
fields and a file upload field. Let's say the behaviour of a trivial front-end from show as below
<form>
<input type="file" name="image">
<input type="text" name="field_1">
<input type="text" name="field_2">
</form>
would result the following post request:
Content-Disposition: form-data; name="image"
Content-Type: application/octet-stream
===CONTENT OF THE IMAGE===
Content-Disposition: form-data; name="field_1"
value of field 1
Content-Disposition: form-data; name="field_2"
value of field 2
But another backend developer insisted that we should wrap all of the fields except the image into a json object so that the request appears like
Content-Disposition: form-data; name="image"
Content-Type: application/octet-stream
===CONTENT OF THE IMAGE===
Content-Disposition: form-data; name="body"
{
"field_1": "value of field 1",
"field_2": "value of field 2"
}
Our backend technology is Spring/Boot, and as a result, the latter design would greatly simplify our controller signature, like
public EmptyResponse acceptFormInput(
#RequestPart EmptyResponse body,
#RequestPart("file") MultipartFile file
)
It appears to me that the Controller should directly reflect what our API looks like. And the controller signature should reflect the API declaration. But that fellow developer argued that the nesting API design is also part of the W3C standard (in draft though), thus should be doable. (W3C plan)
I'm having trouble figuring out which is the common practice in designing such API. Any suggestion is appreciated.