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Fix prepare body stream detection for getattr -based file wrappers

Fix prepare body stream detection for getattr -based file wrappers: a task in requests-pr-diff (Harbor dataset). Title: Fix prepare body stream detection for getattr -based file wrappers

The task

**Title:** Fix `prepare_body` stream detection for `__getattr__`-based file wrappers

Part of sergiopaniego/requests-pr-diff.