fix fetching binary files such as images

[nickurban]

Hello, when I fetch binary files such as images create by code_interpreter, I get a parsing error.

I determined that this is because response.is_a?(String) returns true for binary files such as png.

This PR fixes the parsing error, but I suspect there may be a better way to fix it.

For example, if someone was using non-utf-8 string encoding, this could break.

May need more investigation?

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