[Bug][internal] Multipart FormData crashes on File.Stream or non-UTF-8 bytes

[gsmlg]

Requesting repo: gsmlg-dev/http_fetch on branch main

When a multipart body contains a File.Stream (or any non-binary iodata element, including non-UTF-8 byte strings), HTTP.fetch/2 raises an exception before the request is dispatched. The user-visible result is (ArgumentError) argument error from IO.iodata_to_binary/1.

Root cause (observed)

In lib/http/request.ex, to_httpc_args/1 converts the request body via IO.iodata_to_binary/1. IO.iodata_to_binary/1 only accepts binaries, lists of bytes, and lists thereof — not File.Stream or any custom struct that implements the String.Chars protocol.

HTTP.FormData.append_file/4 accepts a File.stream!/1 (or any iodata-producing enumerable) and embeds the stream into the multipart envelope as iodata. When the request pipeline reaches to_httpc_args/1, the embedded stream is still a struct, and IO.iodata_to_binary/1 rejects it.

Reproduction

form =
  HTTP.FormData.new()
  |> HTTP.FormData.append_field("title", "my doc")
  |> HTTP.FormData.append_file(
    "document",
    "hello.txt",
    File.stream!("hello.txt"),
    "text/plain"
  )

result =
  HTTP.fetch("http://localhost/upload", method: "POST", body: form)
  |> HTTP.Promise.await()

# Expected: %HTTP.Response{status: 200, ...}
# Actual:   {:error, ...} with root cause (ArgumentError) argument error
#           from :erlang.iolist_to_binary/1 inside :io.iodata_to_binary/1

The same crash occurs for any non-UTF-8 byte sequence passed as a body.

Impact

• HTTP.FormData is effectively unusable for the two cases the API advertises: streaming large files from disk and uploading binary blobs.
• Any user that tries to follow the documented File.stream! example hits an immediate crash with no actionable error.

Suggested direction (not requesting a specific implementation)

1. HTTP.Request.to_httpc_args/1 should read the body to a binary before passing it to :httpc for any body that isn't already a binary. For streams, the read should be chunked to avoid loading huge files into memory.
2. Alternatively, HTTP.FormData could materialise the body at append_file time and surface a clearer error if materialisation fails.

Environment

• Elixir 1.18+
• Erlang/OTP 28

Severity

needed (multipart file upload is a documented capability; the current behaviour breaks the primary use case silently with a low-level ArgumentError)