PathLike upload tuples fail with an opaque connection error #196

What happened?

A bare Path works as an upload, but the tuple form accepted by FileTypes does not:

from pathlib import Path

client.sessions.uploads.create(
    "session-id",
    file=("custom.md", Path("README.md"), "text/markdown"),
)

On current main at 5f5274b, this raises:

APIConnectionError: Connection error.

The underlying cause is:

AttributeError: 'PosixPath' object has no attribute 'read'

The same path is shared by certificate, extension, and session uploads.

Why it happens

is_file_content() treats every tuple as terminal file content. That means _transform_file() returns the tuple unchanged before its tuple-handling branch can read the PathLike value inside it. HTTPX later receives the raw Path and tries to call .read() on it.

Expected behavior

A PathLike inside any supported upload tuple should be read just like a bare Path, while preserving the custom filename, content type, and optional headers.

I have a small fix ready that keeps tuples separate from direct file content and adds sync and async coverage for all three supported tuple shapes.