Mint.HTTP1.request

You're seeing just the function request, go back to Mint.HTTP1 module for more information.
Link to this function

request(conn, method, path, headers, body)

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Specs

request(
  t(),
  method :: String.t(),
  path :: String.t(),
  Mint.Types.headers(),
  body :: iodata() | nil | :stream
) :: {:ok, t(), Mint.Types.request_ref()} | {:error, t(), Mint.Types.error()}

See Mint.HTTP.request/5.

In HTTP/1 and HTTP/1.1, you can't open a new request if you're streaming the body of another request. If you try, an error will be returned.