More robust check for upload files in binary mode

[lkuchenb]

This is a minor patch proposal fixing fixing an issue with a recently introduced check for binary file objects in multipart uploads. The currently used check

httpx/httpx/_multipart.py

Lines 125 to 128 in f1157db

	if "b" not in getattr(fileobj, "mode", "b"):
	raise TypeError(
	"Multipart file uploads must be opened in binary mode, not text mode."
	)

misses out on file objects that read and write byte-like objects, but have the mode attribute set to r such as ZipFile.open() which is binary-only.

[lkuchenb]

The StringIO check is now somewhat obsolete, I've changed the order so that the more specific test comes first, but they could probably also be merged into one with a meaningful combined message.

[tomchristie]

do we think it's worth adding an extra test for this case or we happy to accept the pr as-is?

[tomchristie]

Thanks!

[lkuchenb]

Thanks for the review @tomchristie - I didn't think covering this with a test would make much sense since the test would involve intentionally making false assumptions about standard library behavior, not something one would typically want to test IMHO.