[whatwg/fetch] [Suggestion] Add more per-file (custom) header to multipart (#1130)

Some demonstration code
```js
fd = new FormData()
file = new File(['abc'], 'dummy.txt', { type: 'text/plain', lastModified: Date.now() })
fd.set('x', file)
await new Response(fd).text()
```
What's generated:
```
------WebKitFormBoundaryI2aNXnjBifHTBeH1
Content-Disposition: form-data; name="x"; filename="dummy.txt"
Content-Type: text/plain

abc
------WebKitFormBoundaryI2aNXnjBifHTBeH1--
```

---

I think some more per-file headers could enhance the server validation
for instance. it would be useful to know before you receive the hole file how large a per file size is, not the hole formdata `content-length`. So you can abort upload request prematurely if one is sending a too large file. While we are at it, why can't we as well throw in the less useful `Last-Modified` header as well? that is also known on the client side before it's being uploaded.


```
------WebKitFormBoundaryI2aNXnjBifHTBeH1
Content-Disposition: form-data; name="x"; filename="dummy.txt"
Content-Type: text/plain
Content-Length: 3
Last-Modified: Wed, 21 Oct 2015 07:28:00 GMT

abc
------WebKitFormBoundaryI2aNXnjBifHTBeH1--
```

This things could easily be added if you pass in a Blob/File without changing any code. But another idea could also be to allow sending custom headers as well when using the `FormData` api 
```js
// something like
headers = new Headers({
  'Content-Length': 3
})
fd.set('x', file, { headers } )
```
This could allow for a way to [batch](https://www.arangodb.com/docs/stable/http/batch-request.html) a bunch of requests into one request also. if you get a bit creative
I have seen some developer use a custom multipart library to batch multiple request/responses into one 

As opposite to this a way to read individual per-entries headers could also be useful so you can decode it also.

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