docs: clarify filenames for in-memory file uploads

[sylvesterkaczmarek]

Summary

• document that file-like objects such as io.BytesIO are accepted upload inputs
• show the explicit (filename, contents, media type) form for in-memory audio uploads
• explain why APIs that infer formats from file extensions need a meaningful multipart filename
• document the alternative of setting .name on a mutable file-like object

Fixes #2315

Test plan

• git diff --check

This is a documentation-only change; the existing upload transport behavior is unchanged.