Now, the `Client.do_api_query()` method supports sending files to the
API.
This has allowed the implementation of a new method,
`Client.upload_file(file)`. It simply uploads the file set in the
parameter, and returns the API's response (that includes the URI).
Despite the fact that `do_api_query()` supports multiple files as
parameters, `upload_file()` doesn't, because right now the API isn't
capable of managing more than a file in the same request.
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