add handle_file docs

[pngwn]

Description

Adds handle_file documentation to the client README.

Closes: #8476.

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[gradio-pr-bot]

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Install Gradio from this PR

pip install https://gradio-builds.s3.amazonaws.com/446613b862dc35254e34c7d2d8e479022744a783/gradio-4.36.1-py3-none-any.whl

Install Gradio Python Client from this PR

pip install "gradio-client @ git+https://github.com/gradio-app/gradio@446613b862dc35254e34c7d2d8e479022744a783#subdirectory=client/python"

Install Gradio JS Client from this PR

npm install https://gradio-builds.s3.amazonaws.com/446613b862dc35254e34c7d2d8e479022744a783/gradio-client-1.1.1.tgz

[gradio-pr-bot]

🦄 change detected

This Pull Request includes changes to the following packages.

Package	Version
@gradio/client	minor
gradio	minor

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With the following changelog entry.

add handle_file docs

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[freddyaboulton]

Docs look good @pngwn thanks!