API requests with files error on retry with aiohttp 3.7

[Jackenmen]

Summary

If API request with a file (i.e. sending a message with a file) fails and d.py needs to try the request, it errors out

Reproduction Steps

Reliable repro is a bit hard to make as this only happens if d.py needs to retry the API request.
The code with which this can happen can be something like this:

from io import BytesIO
fp = BytesIO(b"foobar")
await ctx.send("message", discord.File(fp, "testfile.txt"))

Expected Results

No error

Actual Results

Traceback:

File "python3.8/site-packages/discord/abc.py", line 915, in send
  data = await state.http.send_files(channel.id, files=[file], allowed_mentions=allowed_mentions,
File "python3.8/site-packages/discord/http.py", line 185, in request
  async with self.__session.request(method, url, **kwargs) as r:
File "python3.8/site-packages/aiohttp/client.py", line 1117, in __aenter__
  self._resp = await self._coro
File "python3.8/site-packages/aiohttp/client.py", line 492, in _request
  req = self._request_class(
File "python3.8/site-packages/aiohttp/client_reqrep.py", line 313, in __init__
  self.update_body_from_data(data)
File "python3.8/site-packages/aiohttp/client_reqrep.py", line 507, in update_body_from_data
  body = body()
File "python3.8/site-packages/aiohttp/formdata.py", line 168, in __call__
  return self._gen_form_data()
File "python3.8/site-packages/aiohttp/formdata.py", line 132, in _gen_form_data
  raise RuntimeError("Form data has been processed already")
RuntimeError: Form data has been processed already

Intents

discord.Intents.all()

System Information

• Python v3.8.8-final
• discord.py v1.6.0-final
• aiohttp v3.7.3
• system info: Windows 10 10.0.19041

Checklist

• I have searched the open issues for duplicates.
• I have shown the entire traceback, if possible.
• I have removed my token from display, if visible.

This happens on aiohttp 3.7 as the FormData object has been changed so that it only can be used once.

Related commit and issues:
aio-libs/aiohttp@d931f10
aio-libs/aiohttp#4351
aio-libs/aiohttp#4345

I know you're already aware of this issue but I figured it might be worth making a tracking issue for this so that it doesn't get lost.