How to place a multipart/related request using browser fetch api?

My api server accepts multipart/related kinds of POST reqeust. But I find no resource which explains how to place one such request using browser fetch api. Almost all links tells how to use FormData , but the content type for form data is not multipart/related, which I seek. I need to upload a JSON along accompanied by various attachment files. How do I make such a fetch request ?

Is there any standard way to do this , like the FormData object ? Or I have to write it manually ?

This is how I expect my output to be

PUT /target/SpaghettiWithMeatballs?new_edits=false HTTP/1.1
Accept: application/json
Content-Length: 1030
Content-Type: multipart/related; boundary="864d690aeb91f25d469dec6851fb57f2"
Host: localhost:5984
User-Agent: CouchDB

--2fa48cba80d0cdba7829931fe8acce9d
Content-Type: application/json

{
    "_attachments": {
        "recipe.txt": {
            "content_type": "text/plain",
            "digest": "md5-R5CrCb6fX10Y46AqtNn0oQ==",
            "follows": true,
            "length": 87,
            "revpos": 7
        }
    },
    "_id": "SpaghettiWithMeatballs",
    "_rev": "7-474f12eb068c717243487a9505f6123b",
    "_revisions": {
        "ids": [
            "474f12eb068c717243487a9505f6123b",
            "5949cfcd437e3ee22d2d98a26d1a83bf",
            "00ecbbc54e2a171156ec345b77dfdf59",
            "fc997b62794a6268f2636a4a176efcd6",
            "3552c87351aadc1e4bea2461a1e8113a",
            "404838bc2862ce76c6ebed046f9eb542",
            "5defd9d813628cea6e98196eb0ee8594"
        ],
        "start": 7
    },
    "description": "An Italian-American delicious dish",
    "ingredients": [
        "spaghetti",
        "tomato sauce",
        "meatballs",
        "love"
    ],
    "name": "Spaghetti with meatballs"
}
--2fa48cba80d0cdba7829931fe8acce9d
Content-Disposition: attachment; filename="recipe.txt"
Content-Type: text/plain
Content-Length: 87

1. Cook spaghetti
2. Cook meetballs
3. Mix them
4. Add tomato sauce
5. ...
6. PROFIT!

--2fa48cba80d0cdba7829931fe8acce9d--

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