Fix generated go client cannot upload file

Created by: dhontecillas

Fixes #8392 (closed) with the naive approach of checking if the body type is **os.File , so it can read its content and avoid the bug of not reading the bytes and having content-length 0.

Tested the fixed code on a separate branch in the same repo used to reproduce the issue : https://github.com/dhontecillas/openapi-generator-go-upload-bug/tree/test_fix (the updated line can be found in https://github.com/dhontecillas/openapi-generator-go-upload-bug/blob/test_fix/pkg/bug/client.go#L415 ). When running the go run . command in that branch, the bug is not reproduced anymore. We get an output of :

The request failed for a different reason: Post "http://127.0.0.1:8000/upload": dial tcp 127.0.0.1:8000: connect: connection refused

That is the expected result as we do not have any server running.

Can you have a look @antihax @grokify @kemokemo @jirikuncar please ? thank you

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